View source for Determination of the day of the week

{{Short description|Methods to calculate the day of the week}}
{{Use dmy dates|date=July 2019|cs1-dates=y}}
The '''determination of the day of the week''' for any date may be performed with a variety of [[algorithm]]s. In addition, [[perpetual calendars]] require no calculation by the user, and are essentially lookup tables.
A typical application is to calculate the [[day of the week]] on which someone was born or a specific event occurred.

==Concepts==
In numerical calculation, the days of the week are represented as weekday numbers. If Monday is the first day of the week, the days may be coded 1 to 7, for Monday through Sunday, as is practiced in [[ISO 8601]]. The day designated with 7 may also be counted as ''0'', by applying the [[Modular arithmetic|arithmetic modulo]] 7, which calculates the remainder of a number after division by 7. Thus, the number 7 is treated as 0, the number 8 as 1, the number 9 as 2, the number 18 as 4, and so on. If Sunday is counted as day 1, then 7 days later (i.e.{{nbs}}day 8) is also a Sunday, and day 18 is the same as day 4, which is a Wednesday since this falls three days after Sunday (i.e.{{nbs}}{{math|18 mod 7 {{=}} 4}}).{{efn|To illustrate this in detail it is useful to visualise a calendar depicting a month beginning on a Sunday, i.e.{{nbs}}the 1st of the month is a Sunday). Counting forward seven days brings us to the 8th, which is also a Sunday. Counting forward another ten days brings us to the 18th, which is a Wednesday. If instead we were to start on Wednesday the 4th (three days after Sunday the 1st), count forward seven days to Wednesday the 11th (three days after Sunday the 8th), then count forward another seven days, we would end up on Wednesday the {{nowrap|18th{{tsp}}{{ndash}}{{tsp}}}}{{nowrap|again{{tsp}}{{ndash}}{{tsp}}}}three days after Sunday the 15th, which itself falls exactly two weeks (two Sundays) after Sunday the 1st.}}

{| class="wikitable"
|-
! Standard
! Monday
! Tuesday
! Wednesday
! Thursday
! Friday
! Saturday
! Sunday
! Usage examples
|-
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| {{tt|%_ISODOWI%, %@ISODOWI[]%}} ([[4DOS]]);<ref name="4DOS_8.00_HELP">{{cite book |title=4DOS 8.00 online help |title-link=4DOS 8.00 |author-first1=Hardin |author-last1=Brothers |author-first2=Tom |author-last2=Rawson |author-link2=Tom Rawson |author-first3=Rex C. |author-last3=Conn |author-link3=Rex C. Conn |author-first4=Matthias R. |author-last4=Paul |author-first5=Charles E. |author-last5=Dye |author-first6=Luchezar I. |author-last6=Georgiev |date=2002-02-27}}</ref> {{tt|DAYOFWEEK()}} ([[HP Prime]])<ref name="Moravia_2015">{{cite web |title=HP Prime - Portal: Firmware update |publisher=Moravia Education |date=2015-05-15 |language=de |url=http://hp-prime.de/de/id/61-firmware-update |access-date=2015-08-28 |url-status=live |archive-url=https://web.archive.org/web/20161105001326/http://hp-prime.de/de/id/61-firmware-update |archive-date=2016-11-05}}</ref>
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| {{tt|[[%NDAY OF WEEK%]]}} ([[NetWare]], [[DR-DOS]]<ref>{{cite book |title=NWDOS-TIPs &mdash; Tips & Tricks rund um Novell DOS 7, mit Blick auf undokumentierte Details, Bugs und Workarounds |author-first=Matthias R. |author-last=Paul |date=1997-07-30 |edition=3rd |version=Release 157 |language=de |url=http://www.antonis.de/dos/dos-tuts/mpdostip/html/nwdostip.htm |access-date=2014-08-06 |url-status=live |archive-url=https://web.archive.org/web/20161104235829/http://www.antonis.de/dos/dos-tuts/mpdostip/html/nwdostip.htm |archive-date=2016-11-04}} (NB. <code>NWDOSTIP.TXT</code> is a comprehensive work on [[Novell DOS 7]] and [[OpenDOS 7.01]], including the description of many undocumented features and internals. It is part of the author's yet larger <code>MPDOSTIP.ZIP</code> collection maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of the <code>NWDOSTIP.TXT</code> file.)</ref>); {{tt|%_DOWI%, %@DOWI[]%}} ([[4DOS]])<ref name="4DOS_8.00_HELP"/>
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The basic approach of nearly all of the methods to calculate the day of the week begins by starting from an "anchor date": a known pair (such as 1 January 1800 as a Wednesday), determining the number of days between the known day and the day to determine, and using arithmetic modulo 7 to find a new numerical day of the week.

One standard approach is to look up (or calculate, using a known rule) the value of the first day of the week of a given century, look up (or calculate, using a method of congruence) an adjustment for the month, calculate the number of leap years since the start of the century, and then add these together along with the number of years since the start of the century, and the day number of the month. Eventually, applying modulo 7 to the day count allows determining the day of the week of that date.<ref>{{cite book |author-last=Richards |author-first=E. G. |date=1999 |title=Mapping Time: The Calendar and Its History |url=https://archive.org/details/mappingtimecalen00rich |url-access=registration |publisher=[[Oxford University Press]]|isbn=978-0-19-850413-9 }}</ref>

Some methods do all the additions first and then cast out sevens, whereas others cast them out at each step, as in [[#Lewis Carroll's method|Lewis Carroll's method]]. Either way can have practical use: the former is easier for calculators and computer programs, whereas the latter is easier for mental calculation. None of the methods given here perform range checks, so unreasonable dates will produce erroneous results.

===Corresponding days===
Every seventh day in a month has the same name as the previous:
{| class="wikitable" 
|-
!Day of<br />the month ||{{mvar|d}}
|-
|00 07 14 21 28 ||0
|-
|01 08 15 22 29 ||1
|-
|02 09 16 23 30 ||2
|-
|03 10 17 24 31 ||3
|-
|04 11 18 25 ||4
|-
|05 12 19 26 ||5
|-
|06 13 20 27 ||6
|}

===Corresponding months===
"Corresponding months" are those months within the calendar year that start on the same day of the week. For example, September and December correspond, because 1 September falls on the same day as 1 December (as there are precisely thirteen 7-day weeks between the two dates). Months can only correspond if the number of days between their first days is divisible by 7, or in other words, if their first days are a whole number of weeks apart. For example, February of a [[common year]] corresponds to March because February has 28 days, a number divisible by 7, 28 days being exactly four weeks.
In a [[leap year]], January and February correspond to different months than in a common year, since adding 29 February means each subsequent month starts a day later.

January corresponds to October in common years and April and July in leap years. February corresponds to March and November in common years and August in leap years. March always corresponds to November, April always corresponds to July, and September always corresponds to December. August does not correspond to any other month in a common year. October doesn't correspond to any other month in a leap year. May and June never correspond to any other month.

In the months table below, corresponding months have the same number, a fact which follows directly from the definition.
{| class="wikitable" style="text-align:center;"
|-
!Common years!!Leap years!!All years!!{{mvar|m}}
|-
| Jan ||     || Oct     || 0
|-
|     ||     || May     || 1
|-
|     || Feb || Aug     || 2
|-
| Feb ||     || Mar Nov || 3
|-
|     ||     || Jun     || 4
|-
|     ||     || Sept Dec || 5
|-
|     || Jan || Apr July || 6
|}

=== Corresponding years ===
{{cleanup list|section|date=March 2021}}
<!--The example given is to be updated every 4 years. Its next update should be in 2027, then 2031, and so on.-->

There are seven possible days that a year can start on, and leap years will alter the day of the week after 29 February. This means that there are 14 configurations that a year can have. All the configurations can be referenced by a [[dominical letter]], but as 29 February has no letter allocated to it, a leap year has two dominical letters, one for January and February and the other (one step back in the alphabetical sequence) for March to December.

2021 is a common year starting on Friday, which means that it corresponds to the 2010 calendar year. The first two months of 2021 correspond to the first two months of 2016. 2022 is a common year starting on Saturday, which means that it corresponds to the 2011 calendar year. The last ten months of 2022 correspond to the last ten months of 2016. 2023 is a common year starting on Sunday, which means that it corresponds to the 2017 calendar year. 2024 is a leap year starting on Monday, which means that it corresponds to the 1996 calendar year. The first two months of 2024 correspond to the first two months of 2018. The last ten months of 2024 correspond to the last ten months of 2019.

Each leap year repeats once every 28 years, and every common year repeats once every 6 years and twice every 11 years. For instance, the last occurrence of a leap year starting on Wednesday was 2020 and the next occurrence will be 2048. Likewise, the next common years starting on Friday will be 2027, 2038, and then 2049. Both of these statements are true unless a leap year is skipped, of which will not happen until 2100.

For details, see the table below:

{| class="wikitable" style="text-align:center"
|-
! Year of the<br />century mod 28 ||{{mvar|y}}
|-
| <span style="color:blue;">00</span> 06 <span style="color:red;">12</span> 17 23 ||0
|-
| 01 07 <span style="color:blue;">12</span> 18 <span style="color:red;">24</span> ||1
|- 
| 02 <span style="color:red;">08</span> 13 19 <span style="color:blue;">24</span> ||2
|-
| 03 <span style="color:blue;">08</span> 14 <span style="color:red;">20</span> 25 ||3
|-
| <span style="color:red;">04</span> 09 15 <span style="color:blue;">20</span> 26 ||4
|-
| <span style="color:blue;">04</span> 10 <span style="color:red;">16</span> 21 27 ||5
|-
| 05 11 <span style="color:blue;">16</span> 22 <span style="color:red;">00</span> ||6
|}

Notes:

* Black means all the months of a common year
* <span style="color:red;">Red</span> means the first two months of a leap year
* <span style="color:blue;">Blue</span> means the last ten months of a leap year

===Corresponding centuries===

{| class="wikitable" style="text-align:center"
|-
!Julian century<br />mod 700||Gregorian century<br />mod 400{{efn|The numbers in the first column are proleptic – the Gregorian calendar was not devised till 1582. See the note beneath the table.}}||Day
|-
|400: 1100 1800 ...||300: 1500 1900 ...||Sun
|-
|300: 1000 1700 ...||||Mon
|-
|200  0900 1600 ...||200: 1800 2200 ...||Tue
|-
|100  0800 1500 ...||||Wed
|-
|700: 1400 2100 ...||100: 1700 2100 ...||Thu{{efn|The Julian century beginning 1 BC would also appear on this line of the table (to the left of 700) but there is no space to include it.}}
|-
|600: 1300 2000 ...||||Fri
|-
|500: 1200 1900 ...||000: 1600 2000 ...||Sat
|}

"Year 000" is, in normal chronology, the year 1 BC (which precedes AD 1). In [[astronomical year numbering]] the year 0 comes between 1 BC and AD 1. In the [[proleptic Julian calendar]], (that is, the Julian calendar as it would have been if it had been operated correctly from the start), 1 BC starts on Thursday. In the [[proleptic Gregorian calendar]], (called such because it wasn't devised until 1582), 1 BC starts on Saturday.

==Tabular methods to calculate the day of the week==
===Complete table: Julian and Gregorian calendars===
For Julian dates before 1300 and after 1999 the year in the table which differs by an exact multiple of 700 years should be used. For Gregorian dates after 2299, the year in the table which differs by an exact multiple of 400 years should be used. The values "{{small|r0}}" through "{{small|r6}}" indicate the remainder when the Hundreds value is divided by 7 and 4 respectively, indicating how the series extend in either direction. Both Julian and Gregorian values are shown 1500–1999 for convenience. Bold figures (e.g., '''04''') denote leap year. If a year ends in 00 and its hundreds are in bold it is a leap year. Thus 19 indicates that 1900 is not a Gregorian leap year, (but '''19''' in the Julian column indicates that it ''is'' a Julian leap year, as are all Julian ''x''00 years). '''20''' indicates that 2000 is a leap year. Use '''Jan''' and '''Feb''' only in leap years.

{| class=wikitable
!colspan=2 style="line-height:10px; border-bottom:none"|<small>Hundreds of years</small>||colspan=4 rowspan=2|Remaining year digits||colspan=5 rowspan=2|Month
!colspan=1 rowspan=2 {{verth|DoW}} ||colspan=1 rowspan=2|#
|-
! style="border-top:none; line-height:10px"|<small>Julian<br />(r ÷ 7)</small>
! style="border-top:none; line-height:10px"|<small>Gregorian<br />(r ÷ 4)</small>
|-
|<small>'''r5'''</small> '''19'''||'''16''' '''20''' <small>'''r0'''</small>||00 06 &nbsp; 17 23||'''28''' 34 &nbsp; 45 51||'''56''' 62 &nbsp; 73 79||'''84''' 90 ||Jan|| || || || Oct||Sa||0
|-
|<small>'''r4'''</small> '''18'''||15 19 <small>r3</small>||01 07 '''12''' 18 ||29 35 '''40''' 46||57 63 '''68''' 74 ||85 91 '''96'''|| || || May|| || ||Su||1
|-
|<small>'''r3'''</small> '''17'''||{{N/A}}||02 &nbsp; 13 19 '''24'''||30 &nbsp; 41 47 '''52'''||58 &nbsp; 69 75 '''80'''||86 &nbsp; 97|| '''Feb'''|| || || Aug|| ||M||2
|-
|<small>'''r2'''</small> '''16'''||18 22 <small>r2</small>||03 '''08''' 14 &nbsp; 25||31 '''36''' 42 &nbsp; 53||59 '''64''' 70 &nbsp; 81||87 '''92''' 98||Feb|| Mar|| || || Nov||Tu||3
|-
|<small>'''r1'''</small> '''15'''||{{N/A}}|| &nbsp; 09 15 '''20''' 26|| &nbsp; 37 43 '''48''' 54|| &nbsp; 65 71 '''76''' 82|| &nbsp; 93 99 || || || Jun|| || ||W||4
|-
|<small>'''r0'''</small> '''14'''||17 21 <small>r1</small>||'''04''' 10 &nbsp; 21 27||'''32''' 38 &nbsp; 49 55||'''60''' 66 &nbsp; 77 83||'''88''' 94 || || || ||Sep ||Dec||Th||5
|-
|<small>'''r6'''</small> '''13'''||{{N/A}}||05 11 '''16''' 22||33 39 '''44''' 50||61 67 '''72''' 78||89 95 ||'''Jan'''|| Apr||  Jul|| || ||F||6
|}

For determination of the day of the week (1 January 2000, Saturday)
*<span style="background-color:#FFFF88">the day of the month: 1 ~ 31 (1)</span>
*<span style="background-color:#FFFF88">the month: (6)</span>
*<span style="background-color:#FF8888">the year: (0)</span>
*<span style="background-color:#88FF88">the century mod 4 for the Gregorian calendar and mod 7 for the Julian calendar (0)</span>.
*<span style="background-color:#BBBBFF">adding 1+6+0+0=7. Dividing by 7 leaves a remainder of 0, so the day of the week is Saturday.</span>

The formula is w = (d + m + y + c) mod 7.

===Revised Julian calendar===
Note that the date (and hence the day of the week) in the Revised Julian and Gregorian calendars is the same from 14 October 1923 to 28 February AD 2800 inclusive and that for large years it may be possible to subtract 6300 or a multiple thereof before starting so as to reach a year which is within or closer to the table.

To look up the weekday of any date for any year using the table, subtract 100 from the year, divide the difference by 100, multiply the resulting quotient (omitting fractions) by seven and divide the product by nine. Note the quotient (omitting fractions). Enter the table with the Julian year, and just before the final division add 50 and subtract the quotient noted above.

The following is an example of calculating the day of the week for 27 January 8315:

{{nowrap|8315 − 6300}} = 2015, {{nowrap|2015 − 100}} = 1915, {{nowrap|1915 ÷ 100}} = 19 remainder 15, {{nowrap|19 × 7}} = 133, {{nowrap|133 ÷ 9}} = 14 remainder }. 2015 is 700 years ahead of 1315, so 1315 is used. From table: for hundreds (13): 6. For remaining digits (15): 4. For month (January): 0. For date (27): 27. {{nowrap|6 + 4 + 0 + 27 + 50 − 14 = 73}}. {{nowrap|73 ÷ 7 = 10 remainder 3}}. Thus, the day of the week is Thursday.

===Dominical letter===
To find the [[dominical letter]], calculate the day of the week for either 1 January or 1 October, and assign a letter with Sunday corresponding to A, Saturday corresponding to B, ... and Monday corresponding to G.

Leap years have two Sunday letters, so for January and February calculate the day of the week for 1 January; and for March to December calculate the day of the week for 1 October.

Leap years are all years which divide exactly by four with the following exceptions:

In the Gregorian calendar – all years which divide exactly by 100 (other than those which divide exactly by 400).

In the Revised Julian calendar – all years which divide exactly by 100 (other than those which give remainder 200 or 600 when divided by 900).

=== The Doomsday ===
{{Main|Doomsday rule}}
{{Expand section|date=July 2025}}
The Doomsday algorithm is an artefact of recreational mathematics.

=== Result ===
The following is a table for finding the day of the week without calculation.

{|class="wikitable" style="text-align:center"
|-
!colspan=5|Index || Mon<br />A || Tue<br />B || Wed<br />C || Thu<br />D || Fri<br />E || Sat<br />F || Sun<br />G !!colspan=18, rowspan=2|Perpetual Gregorian and Julian calendar<br />Use <span style="color:red;">Jan</span> and <span style="color:red;">Feb</span> for <span style="color:red;">leap years</span><br />Date letter in year row for the letter in century row
All the C days are doomsdays
|-
!Julian<br />century!!Gregorian<br />century!!colspan=3|Date
| '''01'''<br />08<br />15<br />22<br />29 || '''02'''<br />09<br />16<br />23<br />30 || '''03'''<br />10<br />17<br />24<br />31 || '''04'''<br />11<br />18<br />25<br /><br />|| '''05'''<br />12<br />19<br />26<br /><br />|| '''06'''<br />13<br />20<br />27<br /><br />|| '''07'''<br />14<br />21<br />28<br /><br />
|-
| <span style="color:red;">12 19</span> || <span style="color:red;">16 20</span> || Apr || Jul || <span style="color:red;">Jan</span> || <span style="color:blue;">G</span> || A || B || C || D || E || F ||01 || 07 ||<span style="color:red;">12</span>|| 18 || || 29 || 35 ||<span style="color:red;">40</span>|| 46 || || 57 || 63 ||<span style="color:red;">68</span>|| 74 || || 85 || 91 ||<span style="color:red;">96</span>
|-
| <span style="color:red;">13 20</span> ||  || Sep || Dec || ||<span style="color:blue;">F</span> || G || A || B || C || D || E || 02 || || 13 || 19 ||<span style="color:red;">24</span>|| 30 || || 41 || 47 ||<span style="color:red;">52</span>|| 58 || || 69 ||75 ||<span style="color:red;">80</span>||86 || || 97
|-
| <span style="color:red;">14 21</span> || 17 21 || |Jun || || || <span style="color:blue;">E</span> || F || G || A || B || C || D || 03 ||<span style="color:red;">08</span>|| 14 || || 25 || 31 ||<span style="color:red;">36</span>|| 42 ||  || 53 || 59 ||<span style="color:red;">64</span>|| 70 || || 81 || 87 ||<span style="color:red;">92</span>|| 98
|-
| <span style="color:red;">15 22</span> ||  || Feb || Mar || Nov || <span style="color:blue;">D</span> || E || F || | G || A || B || C || || 09 || 15 ||<span style="color:red;">20</span>|| 26 || || 37 || 43 ||<span style="color:red;">48</span>|| 54 || || 65 || 71 ||<span style="color:red;">76</span>|| 82 || || 93 || 99
|-
| <span style="color:red;">16 23</span> || 18 22 || Aug || <span style="color:red;">Feb</span> || || <span style="color:blue;">C</span> || D || E || F || G || A || B ||<span style="color:red;">04</span>|| 10 ||  || 21 || 27||<span style="color:red;">32</span>|| 38 || || 49 || 55 ||<span style="color:red;">60</span>|| 66 || || 77 || 83 ||<span style="color:red;">88</span>|| 94 ||
|-
| <span style="color:red;">17 24</span> ||  || May || || || <span style="color:blue;">B</span> || C || D || E || F || G || A || 05 || 11 ||<span style="color:red;">16</span>|| 22 || || 33 || 39 ||<span style="color:red;">44</span>|| 50 || || 61 || 67 ||<span style="color:red;">72</span>|| 78 || || 89 || 95 ||
|-
| <span style="color:red;">18 25</span> || 19 23 || Jan || Oct || || <span style="color:blue;">A</span> || <span style="color:blue;">B</span> || <span style="color:blue;">C</span> || <span style="color:blue;">D</span> || <span style="color:blue;">E</span> || <span style="color:blue;">F</span> || <span style="color:blue;">G</span> || 06 || || 17 || 23 ||<span style="color:red;">28</span>|| 34 || || 45 || 51 ||<span style="color:red;">56</span>|| 62 || || 73 || 79 ||<span style="color:red;">84</span>|| 90 || ||<span style="color:red;">0</span>0
|-
!colspan=2, rowspan=2|[Year/100] || colspan=3|Gregorian<br />century
| <span style="color:red;">20<br />16</span> ||  || 21<br />17 ||  || 22<br />18||  || 23<br />19
!colspan=18, rowspan=2|Year mod 100
|-
!colspan=3|Julian<br />century
| <span style="color:red;">19<br />12</span> || <span style="color:red;">20<br />13</span> || <span style="color:red;">21<br />14</span>  || <span style="color:red;">22<br />15</span>   || <span style="color:red;">23<br />16</span>  ||  <span style="color:red;">24<br />17</span>  || <span style="color:red;">25<br />18</span>
|}

'''Examples:'''
*For common method
:''26 December 1893 (GD)''
December is in row <span style="color:blue;">F</span> and 26 is in column <span style="color:blue;">E</span>, so the letter for the date is C located in row <span style="color:blue;">F</span> and column <span style="color:blue;">E</span>. 93 (year mod 100) is in row <span style="color:blue;">D</span> (year row) and the letter C in the year row is located in column <span style="color:blue;">G</span>. 18 ([year/100] in the Gregorian century column) is in row <span style="color:blue;">C</span> (century row) and the letter in the century row and column <span style="color:blue;">G</span> is B, so the day of the week is Tuesday.
:''13 October 1307 (JD)''
October 13 is a F day. The letter F in the year row (07) is located in column <span style="color:blue;">G</span>. The letter in the century row (13) and column <span style="color:blue;">G</span> is E, so the day of the week is Friday.
:''1 January 2000 (GD)''
January 1 corresponds to G, G in the year row (<span style="color:red;">0</span>0) corresponds to F in the century row (<span style="color:red;">20</span>), and F corresponds to Saturday.

A pithy formula for the method: ''"Date letter (G), letter (G) is in year row (<span style="color:red;">0</span>0) for the letter (F) in century row (<span style="color:red;">20</span>), and for the day, the letter (F) become weekday (Saturday)"''.

====The Sunday letter method====

Each day of the year (other than 29 February) has a letter allocated to it in the recurring sequence ABCDEFG. The series begins with A on 1 January and continues to A again on 31 December. The Sunday letter is the one which stands against all the Sundays in the year. Since 29 February has no letter, this means that the Sunday letter for March to December is one step back in the sequence compared to that for January and February. The letter for any date will be found where the row containing the month (in black) at the left of the "Latin square" meets the column containing the date above the "Latin square". The Sunday letter will be found where the column containing the century (below the "Latin square") meets the row containing the year's last two digits to the right of the "Latin square". For a leap year, the Sunday letter thus found is the one which applies to March to December.

So, for example, to find the weekday of 16 June 2020:

Column "20" meets row "20" at "D". Row "June" meets column "16" at "F". As F is two letters on from D, so the weekday is two days on from Sunday, i.e. Tuesday.

==Mathematical algorithms==
{{hatnote|Note on notation: In the following text, the backslash {{code|\}} is used to indicate a "flooring integer division", and the percent sign {{code|%}} is used to indicate the [[Modulo operation|remainder of such operation]].}}

=== Rata Die ===
The [[Rata Die]] method works by adding up the number of days {{Mvar|d}} that has passed since a date of known day of the week {{Mvar|D}}. The day of-the-week is then given by {{Math|(''D'' + ''d'') mod 7}}, conforming to whatever convention was used to encode {{Mvar|D}}.

For example, the date of 13 August 2009 is 733632 days from 1 January AD 1. Taking the number mod 7 yields 4, hence a Thursday.

===Gauss's algorithm===
[[Carl Friedrich Gauss]] described a method for calculating the day of the week for 1 January in any given year in a handwritten note in a collection of astronomical tables.<ref name="Gauss">{{cite book |author-last=Gauss |author-first=Carl F. |chapter=Den Wochentag des 1. Januar eines Jahres zu finden. Gueldene Zahl. Epakte. Ostergrenze. |title=Werke. herausgegeben von der Koeniglichen Gesellschaft der Wissenschaften zu Goettingen. |date=1981 |publisher=Georg Olms Verlag |location=Hildesheim |isbn=978-3-48704643-3 |pages=206&ndash;207 |edition=2nd}}</ref> He never published it. It was finally included in his collected works in 1927.<ref name="Schwerdtfeger">{{cite web |author-first=Berndt E. |author-last=Schwerdtfeger |title=Gauss' calendar formula for the day of the week |edition=1.4.26 |date=2010-05-07 |url=http://berndt-schwerdtfeger.de/wp-content/uploads/pdf/cal.pdf |access-date=2012-12-23}}</ref> Compared to Rata Die, the result helps simplify the counting of years.

Gauss's method was applicable to the Gregorian calendar. He numbered the weekdays from 0 to 6 starting with Sunday. He defined the following operation.

; Inputs
: Year number {{mvar|A}}, month number {{mvar|M}}, date number {{mvar|D}}.
:
; Output
: Day of year.
; Procedure
:# First determine the day-of-week {{math|''d''<sub>1</sub>}} of 1 January.
:#* For a Gregorian calendar, the weekday is<ref name="Gauss" /> {{indent|4}}{{math|(1 + 5((''A''&minus;1) % 4) + 4((''A''&minus;1) % 100) + 6((''A''&minus;1) % 400)) % 7.}} Alternatively, set {{math|1=''C'' = ''A'' \ 100}}, {{math|1=''Y'' = ''A'' % 100}}, and the value is {{indent|4}}{{math|(1 + 5((''Y''&minus;1)%4) + 3(''Y''&minus;1) + 5(C%4)) % 7.}}
:#* For a Julian calendar, the weekday is {{indent|4}}{{math|(6 + 5((''A''&minus;1)%4) + 3(''A''&minus;1)) % 7}} or {{indent|4}}{{math|(6 + 5((''Y''&minus;1) % 4) + 3(''Y''&minus;1) + 6C) % 7}}.
:# Now determine the month-related offset {{mvar|m}} by using the lookup table with {{mvar|M}}.
:# Return {{math|1=''d'' = (''d''<sub>1</sub> + ''m'' + ''D'') % 7}}.

{|class="wikitable" style="text-align:center;"
|+Table of month offsets
! Months
| Jan || Feb || Mar || Apr || May || Jun || Jul || Aug || Sep || Oct || Nov || Dec
|-
! Common years
| rowspan=2| 0 || rowspan=2| 3 ||  3 || 6 || 1 || 4 || 6 || 2 || 5 || 0 || 3 || 5
|-
! Leap years
|  4 ||  0 || 2 || 5 || 0 || 3 || 6 || 1 || 4 || 6 
|}

The above procedure can be condensed into a single expression for the Gregorian case:
{{indent|4}}{{math|(D + m + 5((''A''&minus;1)%4) + 4((''A''&minus;1)%100) + 6((''A''&minus;1)%400))%7}}

==== Worked example ====
For year number 2000, {{math|1=''A'' − 1 = 1999}}, {{math|1=''Y'' − 1 = 99}} and {{math|1=''C'' = 19}}, the weekday of 1 January is

:{{math|{{=}} (1 + 5(1999%4) + 4(1999%100) + 6(1999%400))%7}}
:{{math|{{=}} (1 + 1 + 4 + 0) % 7}}
:{{math|{{=}} 6}}

:{{math|{{=}} (1 + 5(99%4) + 3 &times; 99 + 5(19%4))%7}}
:{{math|{{=}} (1 + 1 + 3 + 1)%7}}
:{{math|{{=}} 6 {{=}} Saturday.}}

For example, the weekdays for 30 April 1777 and 23 February 1855 are

:{{math|{{=}} (30 + 6 + 5(1776%4) + 4(1776%100) + 6(1776%400))%7}}
:{{math|{{=}} (2 + 6 + 0 + 3 + 6)%7}}
:{{math|{{=}} 3 {{=}}Wednesday}}

and

:{{math|{{=}} (6 + 23 + {{ceil|2.6 &times; 12}} + 5(1854%4) + 4(1854%100) + 6(1854%400))%7}}
:{{math|{{=}} (6 + 2 + 4 + 3 + 6 + 5)%7}}
:{{math|{{=}} 5 {{=}} Friday.}}

==== Explanation and notes ====
The algorithm for the day-of-week of 1 Jan can be proven using modulo arithmetic. The main point is that because {{math|1=365 % 7 = 1}}, each year adds 1 day to the progression. The rest is adjustment for leap year. The century-based versions have {{math|1=36525 % 7 = 6}}.

The table of month offsets show a divergence in February due to the leap year. A common technique (later used by Zeller) is to shift the month to start with March, so that the leap day is at the tail of the counting. In addition, as later shown by Zeller, the table can be replaced with an arithmetic expression.

This formula was also converted into graphical and tabular methods for calculating any day of the week by Kraitchik and Schwerdtfeger.<ref name="Schwerdtfeger"/><ref name="Kraitchik">{{cite book |last=Kraitchik |first=Maurice |title=Mathematical recreations |date=2006 |publisher=[[Dover Publications]] |isbn=978-0-48645358-3 |edition=2nd revised [Dover] |location=Mineola |pages=109&ndash;116 |chapter=Chapter 5: The calendar}}</ref>

==== Disparate variation ====
The following formula is an example of a version without a lookup table. The year is assumed to begin in March, meaning dates in January and February should be treated as being part of the preceding year. The formula for the Gregorian calendar is<ref>{{Cite book |last=Rosen |first=Kenneth H. |title=Elementary Number Theory and Its Applications |year=2011 |publisher=Addison Wesley |isbn=978-0321500311 |pages=134–137 |language=en}}</ref>

: <math>w = \left(d + \lfloor 2.6m - 0.2 \rfloor + y + \left\lfloor\frac{y}{4}\right\rfloor + \left\lfloor\frac{c}{4}\right\rfloor - 2c\right) \bmod 7,</math>
where

* {{mvar|d}} is the day of the month (1 to 31)
* {{mvar|m}} is the shifted month (March = 1,...,February = 12)
* {{mvar|Y}} is the year unless {{mvar|m}} is 11 = January or 12 = February which are considered part of the preceding year, giving {{math|1=''Y'' = year &minus; 1}}
* {{mvar|c}} is the century given by <math>c = \lfloor Y/100 \rfloor</math>
* {{mvar|y}} is the year relative to the century, given by <math>y = Y - 100c</math>, or simply the last 2 digits of {{mvar|Y}}
* {{mvar|w}} is the day of the week (0 = Sunday,...,6 = Saturday)

{| class="wikitable" style="text-align:center;"
|+Table of month offsets (<math>\lfloor 2.6m - 0.2 \rfloor</math>)
! Months
| Mar || Apr || May || Jun || Jul || Aug || Sep || Oct || Nov || Dec || Jan || Feb
|-
! Offset
| 2 || 5 || 7 || 10 || 12 || 15 || 18 || 20 || 23 || 25 || 28 || 31
|}

=== Zeller's algorithm ===
{{main|Zeller's congruence}}
In Zeller's algorithm, the months are numbered from 3 for March to 14 for February. The year is assumed to begin in March; this means, for example, that January 1995 is to be treated as month 13 of 1994.<ref>{{cite web |url=http://www.merlyn.demon.co.uk/zeller-c.htm#Alg |title=The Calendrical Works of Rektor Chr. Zeller: The Day-of-Week and Easter Formulae |work=Merlyn |author-first=J. R. |author-last=Stockton |date=2010-03-19 |access-date=2012-12-19 |archive-date=2013-07-29  |archive-url=https://web.archive.org/web/20130729204547/http://www.merlyn.demon.co.uk/zeller-c.htm#Alg |url-status=dead }}</ref> 
The formula for the Gregorian calendar is<br />
<math display="block">w \equiv \left(\left\lfloor \frac{13(m+1)}{5}\right\rfloor+\left\lfloor \frac{y}{4}\right\rfloor+\left\lfloor\frac{c}{4}\right\rfloor + d+y-2c\right) \bmod 7 </math>
where
* <math>d</math> is the day of the month (1 to 31)
* <math>m</math> is the shifted month (March=3,...January = 13, February=14)
* <math>Y</math> is the year unless <math>m</math> is 13 = January or 14 = February which are considered part of the preceding year giving <math> Y = \textit{year} -1</math>
* <math>c</math> is the century given by <math>c = \lfloor Y/100 \rfloor</math>
* <math>y</math> is the year relative to the century, given by <math>y = Y - 100c</math>, or simply the last 2 digits of <math>Y</math>
* <math>w</math> is the day of week (1 = Sunday,..0 = Saturday)

{|class="wikitable" style="text-align:center;"
|+Table of month offsets (<math>\lfloor 2.6(m + 1)\rfloor</math>)
! Months
| Mar || Apr || May || Jun || Jul || Aug || Sep || Oct || Nov || Dec || Jan || Feb
|-
! Offset
| 10 || 13 || 15 || 18 || 20 || 23 || 26 || 28 || 31 || 33 || 36 || 39
|}

The only difference is one between Zeller's algorithm ({{mvar|Z}}) and the Disparate Gaussian algorithm ({{mvar|G}}), that is {{math|1=''Z'' − ''G'' = 1 = Sunday}}.

: <math> (d + \lfloor(m + 1)2.6\rfloor + y +\lfloor y/4\rfloor + \lfloor c/4\rfloor - 2c) \bmod 7 - (d + \lfloor 2.6m - 0.2\rfloor + y +\lfloor y/4\rfloor + \lfloor c/4\rfloor - 2c) \bmod 7</math>
: <math> = (\lfloor(m + 2 + 1)2.6 - (2.6m - 0.2)\rfloor) \bmod 7 </math> (March = 3 in {{mvar|Z}} but March = 1 in {{mvar|G}})
: <math> = (\lfloor 2.6m + 7.8 - 2.6m + 0.2\rfloor) \bmod 7</math>
: <math> = 8 \bmod 7 = 1</math>

=== Wang's algorithm ===
Wang's algorithm<ref name="Wang">{{cite magazine |url=http://rmm.ludus-opuscula.org/PDF_Files/Wang_Day_5_8(3_2015)_high.pdf | last1=Wang |first1=Xiang-Sheng |title=Calculating the day of the week: null-days algorithm |magazine=Recreational Mathematics Magazine |issue=3 |date=March 2015 |page=5}}</ref> for human calculation of the Gregorian calendar is (the formula should be subtracted by 1 if m is 1 or 2 if the year is a leap year)<br />
<math display="block"> w = \left(d - d_0(m) + y_0 - y_1 + \left\lfloor y_0/4-y_1/2\right\rfloor - 2\left(c\bmod 4 \right)\right) \bmod 7,</math>
where
* {{tmath|y_0}} is the last digit of the year (''units)''
* {{tmath|y_1}} is the second last digit of the year (''tens'')
* {{tmath|c}} is the century, given by <math>c = \lfloor \textit{year}/100 \rfloor</math>
* {{tmath|d}} is the day of the month (1 to 31)
* {{tmath|m}} is the month (January=1,...,December=12)
* {{tmath|w}} is the day of the week (0=Sunday,...,6=Saturday)
* {{tmath|d_0(m)}} is the null-days function (month offset) with values listed in the following table
{| class="wikitable"
|-
! {{mvar|m}} !!colspan=2| {{tmath|d_0(m)}}
|-
| 1 || 1 || A day
|-
| 3 || 5 ||rowspan=3|{{mvar|m}} + 2
|-
| 5 || 7
|-
| 7 || 9
|-
| 9 || 3 ||rowspan=2|{{mvar|m}} + 1
|-
| 11 || 12 
|-
| 2 || 12 ||{{mvar|m}} + 3
|-
| 4 || 2 ||rowspan=5|{{mvar|m}} − 2
|-
| 6 || 4
|-
| 8 || 6
|-
| 10 || 8
|-
| 12 || 10
|}

An algorithm for the Julian calendar can be derived from the algorithm above
<math display="block"> w = \left(d - d_0(m) + y_0 - y_1 + \left\lfloor y_0/4-y_1/2\right\rfloor - c\right) \bmod 7,</math>
where {{tmath|d_0(m)}} is a doomsday.
{| class="wikitable"
|-
! {{mvar|m}} !!colspan=2| {{tmath|d_0(m)}}
|-
| 1 || 3 || C day
|-
| 3 || 7 ||rowspan=3|{{mvar|m}} + 4
|-
| 5 || 9
|-
| 7 || 11
|-
| 9 || 5 ||rowspan=2|{{mvar|m}} − 4
|-
| 11 || 7 
|-
| 2 || 0 || {{mvar|m}} − 2
|-
| 4 || 4||rowspan=5|{{mvar|m}}
|-
| 6 || 6
|-
| 8 || 8
|-
| 10 || 10
|-
| 12 || 12
|}

== Other algorithms ==
=== Schwerdtfeger's method ===
In a partly tabular method by Schwerdtfeger, the year is split into the century and the two digit year within the century. The approach depends on the month. For {{math|<var>m</var> &ge; 3}},
:<math> c = \left\lfloor\frac{y}{100}\right\rfloor \quad \text{and} \quad g = y - 100 c,</math>
so {{math|<var>g</var>}} is between 0 and 99. For {{math|<var>m</var> {{=}} 1,2}},
:<math> c = \left\lfloor\frac{y-1}{100}\right\rfloor \quad \text{and} \quad g = y - 1 - 100 c.</math>
The formula for the day of the week is<ref name="Schwerdtfeger"/>
:<math> w = \left( d + e + f + g + \left\lfloor\frac{g}{4}\right\rfloor \right) \bmod 7,</math>
where the positive modulus is chosen.<ref name="Schwerdtfeger"/>

The value of {{mvar|e}} is obtained from the following table:
{| class="wikitable" style="text-align:center;"
|-
!{{mvar|m}} || 1 || 2 || 3 || 4 || 5 || 6 || 7 || 8 || 9 || 10 || 11 || 12
|-
|{{mvar|e}} || 0 || 3 || 2 || 5 || 0 || 3 || 5 || 1 || 4 || 6 || 2 || 4
|}

The value of {{mvar|f}} is obtained from the following table, which depends on the calendar. For the Gregorian calendar,<ref name="Schwerdtfeger"/>
{| class="wikitable" style="text-align:center;"
|-
!{{math|''c'' mod 4}}  || 0 || 1 || 2 || 3
|-
|{{mvar|f}} || 0 || 5 || 3 || 1
|}
For the Julian calendar,<ref name="Schwerdtfeger"/>
{| class="wikitable" style="text-align:center;"
|-
!{{math|''c'' mod 7}}  || 0 || 1 || 2 || 3 || 4 || 5 || 6
|-
|{{mvar|f}} || 5 || 4 || 3 || 2 || 1 || 0 || 6
|}

===Lewis Carroll's method===
{{missing information|section|transcription in mathematical expressions for easier comparison|date=March 2021}}
Charles Lutwidge Dodgson ([[Lewis Carroll]]) devised a method resembling a puzzle, yet partly tabular in using the same index numbers for the months as in the "Complete table: Julian and Gregorian calendars" above. He lists the same three adjustments for the first three months of non-leap years, one 7 higher for the last, and gives cryptic instructions for finding the rest; his adjustments for centuries are to be determined using formulas similar to those for the centuries table. Although explicit in asserting that his method also works for [[Old Style]] dates, his example reproduced below to determine that "1676, February&nbsp;23" is a Wednesday only works on a Julian calendar which starts the year on January&nbsp;1, instead of March&nbsp;25 as on the "Old Style" [[Julian calendar]].

'''Algorithm:'''<ref name="Dodgson1887">Dodgson, C.L. ([[Lewis Carroll]]). (1887). "To find the day of the week for any given date". ''Nature'', 31&nbsp;March 1887. Reprinted in ''Mapping Time'', pp.&nbsp;299-301.</ref>
<blockquote>
Take the given date in 4&nbsp;portions, viz. the number of centuries, the number of years over, the month, the day of the month.

Compute the following 4&nbsp;items, adding each, when found, to the total of the previous items. When an item or total exceeds&nbsp;7, divide by&nbsp;7, and keep the remainder only.

Century-item: For '[[Old Style]]' (which ended 2&nbsp;September 1752) subtract from 18. For '[[New Style]]' (which began 14&nbsp;September 1752) divide by 4, take overplus [surplus] from 3, multiply remainder by 2.

Year-item: Add together the number of dozens, the overplus, and the number of 4s in the overplus.

Month-item: If it begins or ends with a vowel, subtract the number, denoting its place in the year, from 10. This, plus its number of days, gives the item for the following month. The item for January is "0"; for February or March, "3"; for December, "12".

Day-item: The total, thus reached, must be corrected, by deducting "1" (first adding 7, if the total be "0"), if the date be January or February in a leap year, remembering that every year, divisible by 4, is a Leap Year, excepting only the century-years, in 'New Style', when the number of centuries is not so divisible (e.g. 1800).

The final result gives the day of the week, "0" meaning Sunday, "1" Monday, and so on.
</blockquote>

'''Examples:'''<ref name="Dodgson1887"/>

;1783, September&nbsp;18
17, divided by 4, leaves "1" over; 1 from 3 gives "2"; twice 2 is "4".
83 is 6 dozen and 11, giving 17; plus 2 gives 19, i.e. (dividing by 7) "5". Total 9, i.e. "2"
The item for August is "8 from 10", i.e. "2"; so, for September, it is "2 plus 31", i.e. "5" Total 7, i.e. "0", which goes out.
18 gives "4". Answer, "Thursday".

;1676, February&nbsp;23
16 from 18 gives "2"
76 is 6 dozen and 4, giving 10; plus 1 gives 11, i.e. "4".
Total "6"
The item for February is "3". Total 9, i.e. "2"
23 gives "2". Total "4"
Correction for Leap Year gives "3". Answer, "Wednesday".

Dates before 1752 would in England be given [[Old Style]] with 25&nbsp;March as the [[New Year's Day#History|first day of the new year]]. Carroll's method however assumes 1&nbsp;January as the first day of the year, thus he fails to arrive at the correct answer, namely "Friday".

Had he noticed that ''1676, February 23'' (with 25&nbsp;March as New Year's Day) is actually ''1677, February 23'' (with 1&nbsp;January as New Year's Day), he would have accounted for differing year numbers—just like [[George Washington]]'s birthday differs—between the two calendars. Then his method yields:

;1677 (corrected), February 23 
16 from 18 gives "2" 77 is 6 dozen and 5, giving 11; plus 1 gives 12, i.e. "5". Total "7" The item for February is "3". Total 10, i.e. "3" 23 gives "2". Total "5". Answer, "Friday".

It is noteworthy that those who have republished Carroll's method have failed to point out his error, most notably [[Martin Gardner]].<ref>Martin Gardner. (1996). 
''The Universe in a Handkerchief: Lewis Carroll's Mathematical Recreations, Games, Puzzles, and Word Plays'', pages&nbsp;24-26. Springer-Verlag.</ref>

In 1752, the British Empire abandoned its use of the [[Old Style]] [[Julian calendar]] upon adopting the [[Gregorian calendar]], which has become today's standard in most countries of the world. For more background, see [[Old Style and New Style dates]].

===Methods in computer code===
==== Keith ====
In the [[C language]] expressions below, <code>y</code>, <code>m</code> and <code>d</code> are, respectively, integer variables representing the year (e.g., 1988), month (1–12) and day of the month (1–31). 
<syntaxhighlight lang="c">
(d+=m<3 ? y-- : y-2,23*m/9+d+4+y/4-y/100+y/400) % 7
</syntaxhighlight>
In 1990, Michael Keith and Tom Craver published the foregoing expression that seeks to minimize the number of keystrokes needed to enter a self-contained function for converting a Gregorian date into a numerical day of the week.<ref>
Michael Keith; Tom Craver. (1990). ''The ultimate perpetual calendar?'' Journal of Recreational Mathematics, 22:4, pp.280-282.</ref> It returns <code>0</code> = Sunday, <code>1</code> = Monday, etc. This expression uses a less cumbersome month component than does Zeller's algorithm.

Shortly afterwards, Hans Lachman streamlined their algorithm for ease of use on low-end devices. As designed originally for four-function calculators, his method needs fewer keypad entries by limiting its range either to A.D. 1905–2099, or to historical Julian dates. It was later modified to convert any Gregorian date, even on an [[Abacus#Binary abacus|abacus]]. On [[Motorola 68000#Example code|Motorola 68000]]-based devices, there is similarly less need of either [[processor registers]] or [[opcodes]], depending on the intended design objective.<ref>
The 4-function Calculator; The Assembly of Motorola 68000 Orphans; The Abacus. gopher://sdf.org/1/users/retroburrowers/TemporalRetrology</ref>

====Sakamoto's methods====
The tabular forerunner to Tøndering's algorithm is embodied in the following [[K&R C#K.26R C|K&R C]] function.<ref>"Day-of-week algorithm NEEDED!" news:1993Apr20.075917.16920@sm.sony.co.jp</ref> With minor changes, it was adapted for other [[high level programming language]]s such as [[APL2]].<ref>APL2 IDIOMS workspace: Date and Time Algorithms, line 15. https://www.ibm.com/support/pages/system/files/support/swg/swgdocs.nsf/0/819cd2373bb755d98525716c004c108e/$FILE/APL2IDIOMS.pdf (2002)</ref> Posted by Tomohiko Sakamoto on the comp.lang.c [[Usenet newsgroup]] in 1992, it is accurate for any Gregorian date.<ref>{{cite web|url=https://groups.google.com/d/msg/comp.lang.c/GPA5wwrVnVw/hi2wB0TXGkAJ|website=Google newsgroups:comp.lang.c.|title=Date -&gt; Day of week conversion | date=December 1992|accessdate=21 June 2020}}</ref><ref>{{cite web|url=https://groups.google.com/d/msg/comp.lang.c/IvROHSayPEM/fQ7B2J5wn10J | website=Google newsgroups:comp.lang.c.|title=DOW algorithm|date=1994|accessdate=21 June 2020}}</ref>
<syntaxhighlight lang=c>
dayofweek(y, m, d)	/* 1 <= m <= 12, y > 1752 (in the U.K.) */
{
    static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
    if ( m < 3 )
    {
        y -= 1;
    }
    return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}
</syntaxhighlight>
It returns <code>0</code> = Sunday, <code>1</code> = Monday, etc.

Sakamoto also simultaneously posted a more obfuscated version:
<syntaxhighlight lang=c>
dow(m,d,y) { y -= m<3; return (y+y/4-y/100+y/400+"-bed=pen+mad."[m]+d) % 7; }
</syntaxhighlight>

This version encodes the month offsets in the string and as a result requires a computer that uses standard [[ASCII]] to run the algorithm correctly, reducing its [[Software portability#Effort to port source code|portability]]. In addition, both algorithms omit {{code|int}} [[Data type#Numeric types|type declarations]], which is allowed in the original [[K&R C#K.26R C|K&R C]] but not allowed in [[ANSI C]].

(Tøndering's algorithm is, again, similar in structure to Zeller's congruence and Keith's short code, except that the month-related component is {{code|31*m/12}}. Sakamoto's is somewhere between the Disparate Gaussian and the Schwerdtfeger's algorithm, apparently unaware of the expression form.)

=== Gauss's method in MATLAB ===

<syntaxhighlight lang="matlab">% example date input
y1 = 2022;
m1 = 1;
d1 = 1;

month_offset = [0 3 3 6 1 4 6 2 5 0 3 5]; % common year

% offset if y1 leap year
if mod(y1,4)==0 && mod(y1,100)==0 && mod(y1,400)==0
   month_offset=[0 3 4 0 2 5 0 3 6 1 4 6]; % leap year
end

% Gregor
weekday_gregor = rem( d1+month_offset(m1) + 5*rem(y1-1,4) + 4*rem(y1-1,100) + 6*rem(y1-1,400),7)

% Julian
weekday_julian = rem(6+5*rem(y1-1,4) + 3*(y1-1),7)</syntaxhighlight>

0: Sunday 1: Monday .. 6: Saturday

=== Gauss<nowiki>'</nowiki>s method for Gregorian calendar in Python ===
<syntaxhighlight lang="python3">
from numpy import remainder as rem

def is_leap_year(year: int) -> bool:
    """Determine whether a year is a leap year."""
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
    
def day_of_week(y: int, m: int, d: int) -> str:
    """Return day of week of given date as string,
    using Gauss's algorithm to find it."""
    if is_leap_year(y):
        month_offset = (0, 3, 4, 0, 2, 5, 0, 3, 6, 1, 4, 6)[m - 1]
    else:
        month_offset = (0, 3, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5)[m - 1]
    y -= 1
    wd = int(rem(d + month_offset + 5 * rem(y, 4) + 4 * rem(y, 100) \
             + 6 * rem(y, 400) , 7))
    
    return ("Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat")[wd]
</syntaxhighlight>

==See also==

* {{anl|Buddhist calendar}}
* {{anl|Doomsday rule}}
* {{anl|Julian day}}
* {{anl|Mental Calculation World Cup}} (Has a calendar calculation contest)
* {{anl|Perpetual calendar}}

==References==
===Notes===
{{notelist}}

===Citations===
{{Reflist}}

===Further reading===
{{Refbegin}}
* {{cite book|last=Hale-Evans|first=Ron|chapter=Hack #43: Calculate any weekday|title=Mind performance hacks| year=2006| publisher=O'Reilly|location=Beijing|isbn=9780596101534|pages=[https://archive.org/details/mindperformanceh00hale/page/164 164&ndash;169]|edition=1st|chapter-url-access=registration|chapter-url=https://archive.org/details/mindperformanceh00hale/page/164}}
* {{cite journal|last=Thioux|first=Marc|author2=Stark, David E. |author3=Klaiman, Cheryl |author4= Schultz, Robert T. |title=The day of the week when you were born in 700 ms: Calendar computation in an autistic savant.|journal=Journal of Experimental Psychology: Human Perception and Performance| year=2006| volume=32| issue=5| pages=1155–1168| doi=10.1037/0096-1523.32.5.1155| pmid=17002528 }}
* {{cite book|last=Treffert|first=Darold A.|title=Islands of genius : the bountiful mind of the autistic, acquired, and sudden savant|publisher=Jessica Kingsley|location=London|isbn=9781849058735|chapter=Why calendar calculating?|date=12 October 2011 |pages=63&ndash;66|edition=1. publ., [repr.].}}
{{Refend}}

== External links==
* [http://www.tondering.dk/claus/cal/chrweek.php#calcdow Tøndering's algorithm for both Gregorian and Julian calendars]
* [https://www.angelfire.com/my/zelime/calendar.html "Key Day" method used so as to reduce computation & memorization]
* [http://katzentier.de/_misc/perpetual_calendar.htm Compact tabular method for memorisation, also for the Julian calendar]
* [http://www.tondering.dk/claus/cal/gregorian.php#country When countries changed from the Julian calendar]
* [http://www.recordholders.org/en/records/dates.html World records for mentally calculating the day of the week in the Gregorian Calendar ]
* [http://www.recordholders.org/en/list/mental-calculation-rankings.html National records for finding Calendar Dates]
* [http://www.memoriad.com/index.asp?s=kategoriler&b=kategori-detay&kategoriid=dcacea11f97125360e50694fd11c2ae4&lang=EN World Ranking of Memoriad Mental Calendar Dates] (all competitions combined)
* [http://www5a.biglobe.ne.jp/%257eaccent/calendar/retro.htm Identify the year by given month, day, day of week.] {{Webarchive|url=https://web.archive.org/web/20180104141524/http://www5a.biglobe.ne.jp/~accent/calendar/retro.htm |date=2018-01-04 }}

{{Time measurement and standards}}

[[Category:Days of the week|*determination]]
[[Category:Gregorian calendar]]
[[Category:Julian calendar]]
[[Category:Calendar algorithms]]
[[Category:Articles with example C code]]