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{{Short description|Calendar year containing an additional day}}
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{{Use dmy dates|date=October 2024}}
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A '''leap year''' (also known as an '''intercalary year''' or '''bissextile year''') is a [[calendar year]] that contains an additional day (or, in the case of a [[lunisolar calendar]], a month) compared to a [[common year]]. The 366th day (or 13th month) is added to keep the calendar year synchronised with the [[astronomical year]] or [[seasonal year]].<ref name="Meeus">{{citation|last1 = Meeus | first1=Jean | date = 1998 | title = Astronomical Algorithms | publisher = Willmann-Bell | page=62}}</ref> Since astronomical events and seasons do not repeat in a [[Natural number|whole number]] of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("[[Intercalation (timekeeping)|intercalating]]") an additional day—a '''leap day'''—or month—a '''[[leap month]]'''—into some years, the drift between a civilization's dating system and the physical properties of the [[Solar System]] can be corrected.

An astronomical year lasts slightly less than 365{{sfrac|4}}&nbsp;days. The historic [[Julian calendar]] has three [[common year]]s of 365&nbsp;days followed by a leap year of 366&nbsp;days, by extending February to 29&nbsp;days rather than the common 28. The [[Gregorian calendar]], the world's most widely used civil calendar, makes a further adjustment for the small error in the Julian algorithm. Each leap year has 366&nbsp;days instead of 365. This extra leap day occurs in each year that is a multiple of 4, except for years evenly divisible by 100 but not by 400.

In the lunisolar [[Hebrew calendar]], [[Adar 1|Adar Aleph]], a 13th [[lunar month]], is added seven times every 19&nbsp;years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. In the [[Solar Hijri calendar|Solar Hijri]] and [[Bahá'í calendar]]s, a leap day is added when needed to ensure that the following year begins on the [[March equinox]].

The term ''leap year'' probably comes from the fact that a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, but the day of the week in the 12&nbsp;months following the leap day (from 1{{spaces}}March through 28{{spaces}}February of the following year) will advance two&nbsp;days due to the extra day, thus leaping over one&nbsp;day in the week.<ref>{{citation | first = Douglas | last = Harper | url = http://www.etymonline.com/index.php?term=leap+year&allowed_in_frame=0 | title = leap year | work = Online Etymology Dictionary | date = 2012 | access-date = 15 August 2012 | archive-date = 21 August 2012 | archive-url = https://web.archive.org/web/20120821214540/http://www.etymonline.com/index.php?term=leap+year&allowed_in_frame=0 | url-status = live }}</ref><ref>{{citation |url=http://www.oxforddictionaries.com/us/definition/american_english/leap-year|archive-url=https://web.archive.org/web/20150913171136/http://www.oxforddictionaries.com/us/definition/american_english/leap-year|url-status=dead|archive-date=13 September 2015|title=leap year |website=Oxford US Dictionary | access-date = 6 January 2020}}</ref> For example, since 1{{spaces}}March was a Friday in 2024, was a Saturday in 2025, will be a Sunday in 2026, and a Monday in 2027, but will then "leap" over Tuesday to fall on a Wednesday in 2028.<!-- Please do not change this list until after 2028 /at the very earliest/; leaving it March 1 would do no harm at all, as this is the day that immediately follows February 29 in leap years and February 28 in common years. -->

The length of a day is also occasionally corrected by inserting a [[leap second]] into [[Coordinated Universal Time]] (UTC) because of variations in Earth's [[rotation period]]. Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable.

Leap years can present a problem in computing, known as the [[leap year bug]], when a year is not correctly identified as a leap year or when 29{{spaces}}February is not handled correctly in logic that accepts or manipulates dates.

==Julian calendar==
{{main|Julian calendar}}
On {{nowrap|1 January 45 BC}}, by edict, [[Julius Caesar]] reformed the historic [[Roman calendar]] to make it a consistent [[solar calendar]] (rather than one which was neither strictly lunar nor strictly solar), thus removing the need for frequent [[Intercalation (timekeeping)|intercalary months]]. His rule for leap years was a simple one: add a leap day every 4&nbsp;years. This algorithm is close to reality: a Julian year lasts 365.25{{spaces}}days, a [[mean tropical year]] about 365.2422&nbsp;days, a difference of only {{nowrap|{{midsize|≈}}{{hsp}}11{{midsize|{{frac2|4}}}} min}}.<ref>{{citation |date=2023 |title=Astronomical almanac online glossary |publisher=US Naval Observatory |url=https://aa.usno.navy.mil/faq/asa_glossary#year | access-date=15 August 2024 }}</ref> Consequently, even this Julian calendar drifts out of 'true' by about 3&nbsp;days every 400&nbsp;years. The Julian calendar continued in use unaltered for about 1600&nbsp;years until the Catholic Church became concerned about the widening divergence between the [[March Equinox]] and 21 March, as explained at [[Gregorian calendar]], below.

Prior to Caesar's creation of what would be the Julian calendar, February was already the shortest month of the year for Romans. In the [[Roman calendar]] (after the reform of [[Numa Pompilius]] that added January and February), all months except February had an odd number of days{{snd}}29 or 31. This was because of a [[Religion in ancient Rome|Roman superstition]] that even numbers were unlucky.<ref>{{Citation|title=Why Are There Only 28 Days in February? |encyclopedia=[[Encyclopedia Britannica]] |last=Hogeback |first=Jonathon |url=https://www.britannica.com/story/why-are-there-only-28-days-in-february |access-date=31 May 2023}}</ref> When Caesar changed the calendar to follow the solar year closely, he made all months have 30 or 31&nbsp;days, leaving February unchanged except in leap years.

==Gregorian calendar <span class="anchor" id="Algorithm"></span>==
{{See also|Gregorian calendar}}
[[File:Leap Centuries.jpg|thumb|upright|An image showing which century years are leap years in the Gregorian calendar]]
In the Gregorian calendar, the standard calendar in most of the world,<ref>{{Citation |last1=Dershowitz |first1=Nachum |title=Calendrical calculations |date=2008 |publisher=Cambridge University Press |first2=Edward M. |last2=Reingold |isbn=978-0-521-88540-9 |edition=3rd |location=Cambridge |page=45 |oclc=144768713 |quote="The calendar in use today in most of the world is the Gregorian or '[[Old Style and New Style dates|new-style]]' calendar designed by a commission assembled by Pope Gregory XIII in the sixteenth century."}}</ref> almost every fourth year is a leap year. Each leap year, the month of February has 29 days instead of 28. Adding one extra day in the calendar every four years compensates for the fact that a period of 365 days is shorter than a [[tropical year]] by almost six hours.<ref>{{citation |first1=Ed. K. Lee |last1=Lerner |first2=Brenda W. |last2=Lerner |title=Calendar |encyclopedia=The Gale Encyclopedia of Science |year=2004 |publisher=[[Gale]] |location=Detroit, MI }}</ref> However, this correction is excessive and the [[Gregorian calendar|Gregorian reform]] modified the Julian calendar's scheme of leap years as follows:
<blockquote>Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.<ref>[http://aa.usno.navy.mil/faq/docs/calendars.php Introduction to Calendars] {{Webarchive|url=https://web.archive.org/web/20190613115330/http://aa.usno.navy.mil/faq/docs/calendars.php |date=2019-06-13 }}. (10 August 2017). [[United States Naval Observatory]].</ref></blockquote>

[[File:1800 calendar.jpg|thumb|upright|1800 calendar, showing that February had only 28&nbsp;days]]
Whereas the Julian calendar year incorrectly summarised Earth's tropical year as 365.25 days, the Gregorian calendar makes these exceptions to follow a calendar year of 365.2425 days. This more closely resembles a mean tropical year of 365.2422 days. Over a period of four centuries, the accumulated error of adding a leap day ''every'' four years amounts to about three extra days. The Gregorian calendar therefore omits three leap days every 400 years, which is the length of its ''leap cycle''. This is done by omitting 29 February in the three century years (multiples of 100) that are not multiples of 400.<ref name="usno">{{citation | author = United States Naval Observatory | author-link = United States Naval Observatory | date = 14 June 2011 | url = http://aa.usno.navy.mil/faq/docs/leap_years.php | title = Leap Years | access-date = 9 April 2014 | archive-date = 15 October 2007 | archive-url = https://web.archive.org/web/20071015174504/http://aa.usno.navy.mil/faq/docs/leap_years.php | url-status = dead }}</ref>{{sfn|Lerner|Lerner|2004|p=681}} The years 2000 and 2400 are leap years, but not 1700, 1800, 1900, 2100, 2200, and 2300. By this rule, an entire leap cycle is 400&nbsp;years, which totals 146,097 days, and the average number of days per year is 365&nbsp;+ {{Fraction|4}}&nbsp;− {{Fraction|100}}&nbsp;+ {{Fraction|400}}&nbsp;= 365&nbsp;+ {{Fraction|97|400}}&nbsp;= 365.2425.<ref name="auto">{{citation | last = Richards | first = E. G. | date = 2013 | chapter = Calendars | editor1-first = S. E. | editor1-last = Urban | editor2-first = P. K. | editor2-last = Seidelmann | title = Explanatory Supplement to the Astronomical Almanac | edition =3rd | location = Mill Valley, California | publisher = University Science Books | page = 598 | isbn = 9781891389856}}</ref> This rule could be applied to years before the Gregorian reform to create a [[proleptic Gregorian calendar]],<ref>{{citation | chapter-url = http://www.archive.org/stream/131123ExplanatorySupplementAstronomicalAlmanac/131123-explanatory-supplement-astronomical-almanac#page/n304/mode/1up | last = Doggett | first=L.E. | date=1992 | chapter = Calendars | editor-first = P. K. | editor-last = Seidelmann | title = Explanatory Supplement to the Astronomical Almanac | edition = 2nd | location = Sausalito, California | publisher = University Science Books | pages = 580–1}}</ref> though the result would not match any historical records.

{| width=640
|[[File:Gregoriancalendarleap solstice.svg]]<br />This graph shows the variations in date and time of the [[Solstice|June Solstice]].
|}
The Gregorian calendar was designed to keep the [[March equinox|vernal equinox]] on or close to 21 March, so that the date of [[Easter]] (celebrated on the Sunday after the ecclesiastical [[full moon]] that falls on or after 21 March) remains close to the vernal equinox.<ref>{{citation | first = E. G. | last = Richards | date = 1998 | title = Mapping time: The Calendar and its History | publisher = Oxford University Press | pages = 250–1 | isbn = 0-19-286205-7}}</ref> The "[[Gregorian calendar#Accuracy|Accuracy]]" section of the "[[Gregorian calendar]]" article discusses how well the Gregorian calendar achieves this objective, and how well it approximates the [[tropical year]].

== Leap day in the Julian and Gregorian calendars <span class="anchor" id="Leap day"></span> ==
{{Main|February 29|Bissextus}}
{{Redirect|Leap day|other uses|Leap Day (disambiguation){{!}}Leap Day}}
[[File:Calendar-leapyeardate.jpg|thumb|right|A Swedish pocket calendar from 2008 showing 29 February]]
[[File:Japanese Calendar with Verses by Osman Edwards 1900 February.jpg|thumb|February 1900 calendar showing that 1900 was not a leap year]]

The intercalary day that usually occurs every four&nbsp;years is called leap day and is created by adding an extra day to February. This day is added to the calendar in leap years as a corrective measure because the Earth does not orbit the Sun in precisely 365&nbsp;days. Since about the 15th century, this extra day has been 29 February, but when the Julian calendar was introduced, the leap day was handled differently in two respects. First, leap day fell {{em|within}} February and not at the end: [[Bissextus|24 February was doubled]] to create, strangely to modern eyes, two days both dated 24 February.{{sfnp|Pollard|1940|p=188}} Second, the leap day was simply not counted so that a leap year still had 365&nbsp;days.{{sfnp|Cheney|2000|loc=p 145, footnote 1}}

===Early Roman practice===
{{further|Julian calendar#Motivation}}
The early [[Roman calendar]] was a lunisolar one that consisted of 12&nbsp;months, for a total of 355&nbsp;days. In addition, a 27- or 28-day [[intercalary month]], the {{lang|la|[[Mercedonius|Mensis Intercalaris]]}}, was sometimes inserted into February, at the first or second day after the {{lang|la|[[Terminus (mythology)|Terminalia]]}} {{lang|la|a. d. VII Kal. Mar.}} (23 February), to resynchronise the lunar and solar cycles. The remaining days of Februarius were discarded. This intercalary month, named {{lang|la|Intercalaris}} or {{lang|la|[[Mercedonius]]}}, contained 27&nbsp;days. The religious festivals that were normally celebrated in the last five&nbsp;days of February were moved to the last five&nbsp;days of Intercalaris. The lunisolar calendar was abandoned about 450&nbsp;BC by the {{lang|la|[[decemviri]]}},<ref>According to [[Christian Ludwig Ideler]] (1825)</ref> who implemented the [[Roman calendar|Roman Republican calendar]], used until 46&nbsp;BC. The days of these calendars were counted down (inclusively) to the next named day, so 24 February was {{lang|la|ante diem sextum Kalendas Martias}} ["the sixth day before the calends of March"] often abbreviated {{lang|la|a. d. VI Kal. Mart.}} [[Ancient Rome|The Romans]] counted days inclusively in their calendars, so this was the fifth day before 1 March when counted in the modern exclusive manner (i.e., not including both the starting and ending day).<ref name=Key>{{citation |first = Thomas Hewitt |last = Key |url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Calendarium.html |title=Calendarium |publisher=University of Chicago| orig-year = 1875 |date=2013 |quote=the intermediate days are in all cases reckoned backward upon the Roman principle already explained of counting both extremes.}}</ref> Because only 22 or 23&nbsp;days were effectively added, not a full [[lunation]], the calends and ides of the Roman Republican calendar were no longer associated with the new moon and full moon.{{citation needed|date=February 2024}}

===Julian reform===
In Caesar's revised calendar, there was just one intercalary day{{snd}}nowadays called the leap day{{snd}}to be inserted every fourth year, and this too was done after 23 February. To create the intercalary day, the existing {{lang|la|ante diem sextum Kalendas Martias}} (sixth day (inclusive: i.e., what we would call the fifth day before) before the {{lang|la|[[Kalends]]}} (first day) of March, i.e., what we would call 24 February) was doubled,{{sfnp|Pollard|1940|p=186}} producing {{lang|la|ante diem bis sextum Kalendas Martias}} [a second sixth day before the ''Kalends''. This {{lang|la|bis sextum}} ("twice sixth") was rendered in later languages as "[[bissextile]]": the "bissextile day" is the leap day, and a "bissextile year" is a year which includes a leap day.{{sfnp|Cheney|2000|loc=Page 145, Footnote 1}} This second instance of the sixth day before the Kalends of March was inserted in calendars between the "normal" fifth and sixth days. By legal fiction, the Romans treated both the first "sixth day" and the additional "sixth day" before the Kalends of March as one day. Thus a child born on either of those days in a leap year would have its first birthday on the following sixth day before the Kalends of March. In a leap year in the original Julian calendar, there were indeed two&nbsp;days both numbered 24 February. This practice continued for another fifteen to seventeen centuries, even after most countries had adopted the Gregorian calendar.{{citation needed|date=February 2024}}

For legal purposes, the two&nbsp;days of the {{lang|la|bis sextum}} were considered to be a single day, with the second sixth being intercalated; but in common practice by the year 238, when [[Censorinus]] wrote, the intercalary day was followed by the last five&nbsp;days of February, ''a.'' ''d.'' ''VI'', ''V'', ''IV'', ''III'', and {{lang|la|pridie Kal. Mart.}} (the days numbered 24, 25, 26, 27, and 28 from the beginning of February in a common year), so that the intercalated day was the ''first'' of the doubled pair. Thus the intercalated day was effectively inserted between the 23rd and 24th days of February. All later writers, including [[Macrobius]] about 430, [[Bede]] in 725, and other medieval [[computus|computists]] (calculators of Easter), continued to state that the bissextum (bissextile day) occurred before the last five&nbsp;days of February.{{citation needed|date=December 2022}}

In England, the Church and civil society continued the Roman practice whereby the leap day was simply not counted, so that a leap year was only reckoned as 365&nbsp;days. [[Henry III of England|Henry III]]'s 1236 {{lang|la|[[Statute De Anno et Die Bissextili]]}}{{efn|''Statute concerning [the] leap year and leap day''<br />The day of the leap year, and the day before, shall be holden for one day.<ref name=Bissextili>{{citation |last1=Ruffhead |first1=Owen |title=The Statutes at Large, from Magna Charta to the End of the Last Parliament |date=1763 |publisher=Mark Basket |page=20 |url=https://archive.org/details/statutesatlargef01grea/page/20/mode/1up |access-date=21 October 2021}} (21 Hen, III)</ref>}} instructed magistrates to treat the leap day and the day before as one day.<ref name=Bissextili />{{sfnp|Cheney|2000|loc=Page 145, Footnote 1}} The practical application of the rule is obscure. It was regarded as in force in the time of the famous lawyer Sir [[Edward Coke]] (1552–1634) because he cites it in his ''[[Institutes of the Lawes of England]]''. However, Coke merely quotes the Act with a short translation and does not give practical examples.
{{blockquote|text= ... and by (b) the statute {{lang|la|de anno bissextili}}, it is provided, {{lang|la|quod computentur dies ille excrescens et dies proxime præcedens pro unico dii}}, so as in computation that day [[wiktionary:excrescent|excrescent]] is not accounted.<ref>{{cite book | title = First Part of the [[Institutes of the Lawes of England]] | date = 1628 | chapter-url= https://archive.org/details/firstpartofinsti011628coke/page/116/mode/1up?view=theater | chapter = Cap. 2, ''Of Villenage'' | page = [https://archive.org/details/firstpartofinsti011628coke/page/n287/mode/2up?view=theater 136 left (Sect. 202, line 3)] | first = Edward |last=Coke | author-link = Edward Coke }}</ref>}}

===29 February===
Replacement (by 29 February) of the awkward practice of having two days with the same date appears to have evolved by custom and practice; the etymological origin of the term "bissextile" seems to have been lost.{{sfnp|Pollard|1940|p=188}} In England in the fifteenth century, "29 February" appears increasingly often in legal documents{{snd}}although the records of the proceedings of the [[House of Commons of England]] continued to use the old system until the middle of the sixteenth century.{{sfnp|Pollard|1940|p=188}} It was not until the passage of the [[Calendar (New Style) Act 1750]] that 29 February was formally recognised in British law.<ref>{{Citation |url=https://archive.org/details/in.ernet.dli.2015.69034/page/n226/mode/1up |title=The Statutes at Large: from the 23rd to the 26th Year of King George II |publisher=Charles Bathurst |year=1765 |editor-last=Pickering |editor-first=Danby |editor-link=Danby Pickering |volume=20 |location=Cambridge |access-date=28 January 2020 |page=194}} (calendar at the end of the Act)</ref>{{efn|Though it appears in earlier Government proclamations, such as one of 1619.<ref>{{Citation |last=Bond |first=John James |url=https://archive.org/details/handybookrulesa00bondgoog/page/n6/mode/2up |title=Handy Book of Rules and Tables for Verifying Dates With the Christian Era Giving an Account of the Chief Eras and Systems Used by Various Nations...' |chapter=Preface |page=[https://archive.org/details/handybookrulesa00bondgoog/page/n22/mode/2up xix] |edition=4th|publisher=George Bell & Sons |year=1875 |location=London |author-link=John James Bond }}</ref>}}

===Liturgical practices===
[[File:MissaleLeapYear.jpg|thumb|right|In the older [[Roman Missal]], feast days falling on or after 24 February are celebrated one day later in a leap year.]]
In the [[liturgical calendar]] of the Christian churches, the placement of the leap day is significant because of the date of the feast of [[Saint Matthias]], which is defined as the sixth day before 1 March (counting inclusively). The Church of England's ''[[Book of Common Prayer]]'' was still using the "two days with the same date" system in its 1542 edition;<ref>{{Citation |last1=Campion |first1=Rev W M |url=https://archive.org/details/prayerbookinter01unkngoog/page/n50/mode/2up |title=The Prayer Book interleaved |last2=Beamont |first2=Rev W J |date=1870 |location=London |publisher=Rivingtons |page=31 |via=[[Archive.org]]}}</ref> it first included a calendar which used entirely consecutive day counting from 1662 and showed leap day as falling on 29 February.<ref name="Baskerville">{{Citation |last=Church of England |url=https://archive.org/details/bookcommonpraye00chur/page/n25/mode/1up |title=Book of Common Prayer |publisher=[[John Baskerville]] |year=1762 |location=Cambridge |orig-year=1662 |via=[[Archive.org]]}}</ref> In the 1680s, the Church of England declared 25 February to be the feast of St Matthias.{{sfnp|Cheney|2000|p=8}}
Until 1970, the [[Roman Catholic Church]] always celebrated the feast of Saint Matthias on {{lang|la|a. d. VI Kal. Mart.}}, so if the days were numbered from the beginning of the month, it was named 24 February in common years, but the presence of the {{lang|la|bissextum}} in a bissextile year immediately before {{lang|la|a. d. VI Kal. Mart.}} shifted the latter day to 25 February in leap years, with the [[Vigil]] of St. Matthias shifting from 23 February to the leap day of 24 February. This shift did not take place in pre-Reformation Norway and Iceland; [[Pope Alexander III]] ruled that either practice was lawful.<ref>[[Liber Extra]], 5. 40. 14. 1</ref> Other feasts normally falling on 25–28 February in common years are also shifted to the following day in a leap year (although they would be on the same day according to the Roman notation). The practice is still observed by those who use the older calendars.{{citation needed|date=February 2024}}

In the [[Eastern Orthodox Church]], the feast of St. [[John Cassian]] is celebrated on 29 February, but he is instead commemorated at Compline on 28 February in non-leap years. The feast of St. Matthias is celebrated in August, so leap years do not affect his commemoration, and, while the feast of the [[Beheading of John the Baptist#Related feasts|First and Second Findings of the Head of John the Baptist]] is celebrated on 24 February, the Orthodox church calculates days from the beginning of the current month, rather than counting down days to the Kalends of the following month, this is not affected. Thus, only the feast of St. John Cassian and any movable feasts associated with the Lenten or Pre-Lenten cycles are affected.

===Folk traditions===
[[File:Bob Satterfield cartoon about leap year traditions.jpg|right|thumb|upright|A [[spinster]] eagerly awaits the upcoming leap day, in this 1903 cartoon by [[Bob Satterfield (cartoonist)|Bob Satterfield.]]]]
In Ireland and Britain, it is a [[tradition]] that women may [[Proposal of marriage|propose marriage]] only in leap years. While it has been claimed that the tradition was initiated by [[Saint Patrick]] or [[Brigid of Kildare]] in 5th century Ireland, this is dubious, as the tradition has not been attested before the 19th century.<ref>{{citation | last1 = Mikkelson |first1=B. | last2 = Mikkelson | first2 = D.P. | date = 2010 | url = http://www.snopes.com/oldwives/february29.asp | title = The Privilege of Ladies | website = The Urban Legends Reference Pages | publisher = snopes.com}}</ref> Supposedly, a 1288 law by Queen [[Margaret, Maid of Norway|Margaret of Scotland]] (then age five and living in Norway), required that fines be levied if a marriage proposal was refused by the man; compensation was deemed to be a pair of leather gloves, a single rose, £1, and a kiss.<ref>{{Cite web |last=Ha-Redeye |first=Omar |date=28 February 2016 |title=The Leap Year Proposal Law |url=https://www.slaw.ca/2016/02/28/the-leap-year-proposal-law/ |access-date=8 April 2024 |website=Slaw |language=en-US}}</ref>{{efn|Virtually no laws of Margaret survive. Indeed, none concerning her subjects are recorded in the twelve-volume ''Acts of the Parliaments of Scotland'' (1814–75) covering the period 1124–1707 (two laws concerning young Margaret herself are recorded on pages 424 & 441–2 of volume I).}} In some places the tradition was tightened to restricting female proposals to the modern leap day, 29 February, or to the medieval (bissextile) leap day, 24 February.{{citation needed|date=December 2022}}

According to Felten: "A play from the turn of the 17th century, 'The Maydes Metamorphosis,' has it that 'this is leape year/women wear breeches.' A few hundred years later, breeches wouldn't do at all: Women looking to take advantage of their opportunity to pitch woo were expected to wear a scarlet [[petticoat]]{{snd}}fair warning, if you will."<ref>{{citation | last = Felten | first = E. | date = 23 February 2008 | url = https://www.wsj.com/articles/SB120371485815386581 | title = The Bissextile Beverage | website = Wall Street Journal | access-date = 12 August 2017 | archive-date = 12 August 2017 | archive-url = https://web.archive.org/web/20170812135932/https://www.wsj.com/articles/SB120371485815386581 | url-status = live }}</ref>

In Finland, the tradition is that if a man refuses a woman's proposal on leap day, he should buy her the fabrics for a skirt.<ref>{{citation | last = Hallett | first = S. | date = 29 February 2012 | url = http://www.huffingtonpost.com/2012/02/27/leap-year-proposal-tradition_n_1305525.html | title = Leap Year Proposal: What's The Story Behind It? | website = Huffington Post | access-date = 21 December 2015 | archive-date = 21 October 2014 | archive-url = https://web.archive.org/web/20141021183428/http://www.huffingtonpost.com/2012/02/27/leap-year-proposal-tradition_n_1305525.html | url-status = live }}</ref>

In France, since 1980, a satirical newspaper titled ''[[La Bougie du Sapeur]]'' is published only on leap year, on 29 February.<ref>{{cite web|url=http://www.france24.com/en/20120229-mediawatch-bougie-du-sapeur-newspaper-once-four-years|title=La Bougie du Sapeur: the world's least frequent newspaper|last=Creedon|first=James|date=29 February 2012|work=[[France 24]]|archive-url=https://web.archive.org/web/20120404003951/http://www.france24.com/en/20120229-mediawatch-bougie-du-sapeur-newspaper-once-four-years|archive-date=4 April 2012|url-status=live|access-date=4 June 2024}}</ref>

In Greece, marriage in a leap year is considered unlucky.<ref>{{citation | url = http://www.anagnosis.gr/index.php?la=eng&pageID=151 | title = A Greek Wedding | website = Anagnosis Books | access-date = 12 January 2012 | archive-date = 10 February 2012 | archive-url = https://web.archive.org/web/20120210092205/http://www.anagnosis.gr/index.php?pageID=151&la=eng | url-status = live }}</ref> One in five engaged couples in Greece will plan to avoid getting married in a leap year.<ref>{{citation | url = http://www.developingteachers.com/tips/pasttips63.htm | title = Teaching Tips 63 | website = Developing Teachers | access-date = 12 January 2012 | archive-date = 2 March 2012 | archive-url = https://web.archive.org/web/20120302215023/http://www.developingteachers.com/tips/pasttips63.htm | url-status = dead }}</ref>

In February 1988 the town of [[Anthony, Texas]], declared itself the "leap year capital of the world", and an international leapling birthday club was started.<ref>{{citation|url=http://www.timeanddate.com/date/leap-year-capital.html|title=Anthony – Leap Year Capital of the World|publisher=Time and Date|date=2008|access-date=6 November 2011|archive-date=9 November 2011|archive-url=https://web.archive.org/web/20111109194902/http://www.timeanddate.com/date/leap-year-capital.html|url-status=live}}</ref>

<gallery mode="packed" widths="350px" caption="1908 postcards">
File:PostcardLeapYearBeCarefulClara1908.jpg|Woman capturing man with butterfly-net
File:PostcardLeapYearMaidensAre1908.jpg|Women eagerly awaiting the coming leap year
File:PostcardTheMaidensVowIn1908.jpg|Histrionically preparing
</gallery>

===Birthdays===
A person born on 29 February may be called a "leapling" or a "leaper".<ref>{{citation | date = 28 February 2012 | article = 29 February: 29 things you need to know about leap years and their extra day | title = Mirror | access-date = 7 December 2015 | url = https://www.mirror.co.uk/news/uk-news/leap-years-29-things-you-746716 | archive-date = 2 January 2016 | archive-url = https://web.archive.org/web/20160102001559/http://www.mirror.co.uk/news/uk-news/leap-years-29-things-you-746716 | url-status = live }}</ref> In common years, they celebrate their [[birthday]]s on 28 February or 1 March.

Technically, a leapling will have fewer ''birthday anniversaries'' than their age in years. This phenomenon may be exploited for dramatic effect when a person is declared to be only a quarter of their actual age, by counting their leap-year birthday anniversaries only. For example, in [[Gilbert and Sullivan]]'s 1879 [[comic opera]] ''[[The Pirates of Penzance]]'', Frederic (the pirate apprentice) discovers that he is [[Indentured servitude|bound to serve]] the pirates until his 21st ''birthday'' (that is, when he turns 88&nbsp;years old, since 1900 was not a leap year) rather than until his 21st ''year''.

For legal purposes, legal birthdays depend on how local laws count time intervals.

==== Taiwan ====
{{Wikisource|Civil Code Part I General Principles}}
The Civil Code of [[Taiwan]] since 10 October 1929,<ref>{{citation |title=Legislative History of the Civil Code of the Republic of China |url=http://law.moj.gov.tw/Eng/LawClass/LawHistory.aspx?PCode=B0000001 |access-date=19 July 2011 |archive-date=28 December 2016 |archive-url=https://web.archive.org/web/20161228131414/http://law.moj.gov.tw/Eng/LawClass/LawHistory.aspx?PCode=B0000001 |url-status=live }}</ref> implies that the legal birthday of a leapling is 28 February in common years:

{{Blockquote|If a period fixed by weeks, months, and years does not commence from the beginning of a week, month, or year, it ends with the ending of the day which precedes the day of the last week, month, or year which corresponds to that on which it began to commence. But if there is no corresponding day in the last month, the period ends with the ending of the last day of the last month.<ref>{{citation | article = Article 121 Civil Code | url = http://law.moj.gov.tw/Eng/LawClass/LawAll.aspx?PCode=B0000001 | title = Part I General Principles of the Republic of China | access-date = 2011-07-19 | archive-date = 2021-03-04 | archive-url = https://web.archive.org/web/20210304113355/https://law.moj.gov.tw/Eng/LawClass/LawAll.aspx?PCode=B0000001 | url-status = live }}</ref>}}

==== Hong Kong ====
Since 1990 non-retroactively, [[Hong Kong]] considers the legal birthday of a leapling 1 March in common years:<ref>{{citation |title=Age of Majority (Related Provisions) Ordinance (Ch. 410 Sec. 5) |date=30 June 1997 |url=http://www.legislation.gov.hk/blis_ind.nsf/E1BF50C09A33D3DC482564840019D2F4/22F10EC537B1BEBAC825648300339AA3?OpenDocument |publisher=Hong Kong Department of Justice |access-date=19 July 2011 |archive-date=18 May 2013 |archive-url=https://web.archive.org/web/20130518152417/http://www.legislation.gov.hk/blis_ind.nsf/E1BF50C09A33D3DC482564840019D2F4/22F10EC537B1BEBAC825648300339AA3?OpenDocument |url-status=live }} (Enacted in 1990).</ref>
{{blockquote|
# The time at which a person attains a particular age expressed in years shall be the commencement of the anniversary corresponding to the date of [their] birth.
# Where a person has been born on February 29 in a leap year, the relevant anniversary in any year other than a leap year shall be taken to be March 1.
# This section shall apply only where the relevant anniversary falls on a date after the date of commencement of this Ordinance.}}

====UK====
In the UK 1 March is considered to be a leapling's legal birthday.<ref>{{cite web |title=Leap day birthdays: 'How old are you really?' |url=https://www.bbc.com/news/uk-england-51176188 |publisher=BBC |access-date=3 September 2024 |date=28 February 2020}}</ref>

===Revised Julian calendar===
The [[Revised Julian calendar]] adds an extra day to February in years that are multiples of four, except for years that are multiples of 100 that do not leave a remainder of 200 or 600 when divided by 900. This rule agrees with the rule for the Gregorian calendar until 2799. The first year that dates in the Revised Julian calendar will not agree with those in the Gregorian calendar will be 2800, because it will be a leap year in the Gregorian calendar but not in the Revised Julian calendar.

This rule gives an average year length of 365.242222&nbsp;days. This is a very good approximation to the ''mean'' tropical year, but because the ''vernal equinox'' year is slightly longer, the Revised Julian calendar, for the time being, does not do as good a job as the Gregorian calendar at keeping the vernal equinox on or close to 21 March.

==Baháʼí calendar==
{{main | Baháʼí calendar }}
The Baháʼí calendar is a solar calendar composed of 19 months of 19&nbsp;days each (361&nbsp;days). Years begin at Naw-Rúz, on the [[vernal equinox (Northern Hemisphere)|vernal equinox]], on or about 21 March. A period of intercalary days, called [[Ayyam-i-Ha]], is inserted before the 19th month. This period normally has four&nbsp;days, but an extra day is added when needed to ensure that the following year starts on the [[vernal equinox (Northern Hemisphere)|vernal equinox]]. This is calculated and known years in advance.

== Bengali, Indian and Thai calendars ==
The Revised [[Bengali Calendar]] of Bangladesh and the [[Indian National Calendar]] organise their leap years so that every leap day is close to 29 February in the Gregorian calendar and vice versa. This makes it easy to convert dates to or from Gregorian.

The [[Thai solar calendar]] uses the [[Buddhist Era]] (BE) but has been synchronised with the Gregorian since [[AD]]&nbsp;1941.

==Chinese calendar==
The [[Chinese calendar]] is [[lunisolar calendar|lunisolar]], so a leap year has an extra month, often called an ''embolismic'' month after the Greek word for it. In the Chinese calendar, the [[leap month]] is added according to a rule which ensures that month 11 is always the month that contains the northern winter [[solstice]]. The intercalary month takes the same number as the preceding month; for example, if it follows the second month (二月) then it is simply called "leap second month" i.e., {{lang-zh|t=閏二月|s=闰二月|p=rùn'èryuè}}.

==Hebrew calendar==<!-- This section is linked from [[Jewish holiday]] -->
The Hebrew calendar is lunisolar with an [[embolismic month]]. This extra month is called ''Adar Rishon'' ([[Adar 1|first Adar]]) and is added before ''[[Adar]]'', which then becomes ''Adar Sheini'' ([[Adar 2|second Adar]]). According to the [[Metonic cycle]], this is done seven times every nineteen years (specifically, in years 3, 6, 8, 11, 14, 17, and 19). This is to ensure that [[Passover]] ({{Transliteration|he|''Pesah''}}) is always in the spring as required by the [[Torah]] (Pentateuch) in many verses<ref>Exodus 23:15, Exodus 34:18, Deuteronomy 15:1, Deuteronomy 15:13</ref> relating to Passover.

In addition, the Hebrew calendar has postponement rules that postpone the start of the year by one or two&nbsp;days. These postponement rules reduce the number of different combinations of year length and starting [[Week-day names|days of the week]] from 28 to 14, and regulate the location of certain religious holidays in relation to the [[Shabbat|Sabbath]]. In particular, the first day of the Hebrew year can never be Sunday, Wednesday, or Friday. This rule is known in Hebrew as "{{Transliteration|he|''lo adu rosh''}}" ({{lang|he|לא אד״ו ראש}}), i.e., "Rosh [ha-Shanah, first day of the year] is not Sunday, Wednesday, or Friday" (as the Hebrew word {{Transliteration|he|''adu''}} is written by three [[Hebrew alphabet|Hebrew letters]] signifying Sunday, Wednesday, and Friday). Accordingly, the first day of Passover is never Monday, Wednesday, or Friday. This rule is known in Hebrew as "{{Transliteration|he|''lo badu Pesah''}}" ({{lang|he|לא בד״ו פסח}}), which has a double meaning&nbsp;— "Passover is not a legend", but also "Passover is not Monday, Wednesday, or Friday" (as the Hebrew word {{Transliteration|he|''badu''}} is written by three Hebrew letters signifying Monday, Wednesday, and Friday).

One reason for this rule is that [[Yom Kippur]], the holiest day in the Hebrew calendar and the tenth day of the Hebrew year, now must never be adjacent to the weekly Sabbath (which is Saturday), i.e., it must never fall on Friday or Sunday, in order not to have two adjacent Sabbath days. However, Yom Kippur can still be on Saturday. A second reason is that [[Hoshana Rabbah]], the 21st day of the Hebrew year, will never be on Saturday. These rules for the Feasts do not apply to the years from the Creation to the deliverance of the Hebrews from Egypt under Moses. It was at that time (cf.&nbsp;Exodus 13) that the God of Abraham, Isaac and Jacob gave the Hebrews their "Law" including the days to be kept holy and the feast days and Sabbaths.

Years consisting of 12&nbsp;months have between 353 and 355&nbsp;days. In a {{Transliteration|he|''k'sidra''}} ("in order") 354-day year, months have alternating 30 and 29&nbsp;day lengths. In a {{Transliteration|he|''chaser''}} ("lacking") year, the month of [[Kislev]] is reduced to 29&nbsp;days. In a {{Transliteration|he|''malei''}} ("filled") year, the month of [[Marcheshvan]] is increased to 30&nbsp;days. 13-month years follow the same pattern, with the addition of the 30-day Adar Alef, giving them between 383 and 385&nbsp;days.

==Islamic calendars==
The observed and calculated versions of the lunar [[Islamic calendar]] do not have regular leap days, even though both have lunar months containing 29 or 30&nbsp;days, generally in alternating order. However, the [[tabular Islamic calendar]] used by Islamic astronomers during the Middle Ages and still used by some Muslims does have a regular leap day added to the last month of the lunar year in 11&nbsp;years of a 30-year cycle.<ref name=timeanddateIslamicCalendar>{{citation | url = http://www.timeanddate.com/date/islam-leap-year.html | title = The Islamic leap year | date = n.d. | publisher = Time and Date AS | access-date = 29 February 2012 | archive-date = 3 March 2020 | archive-url = https://web.archive.org/web/20200303004338/http://www.timeanddate.com/date/islam-leap-year.html | url-status = live }}</ref> This additional day is found at the end of the last month, [[Dhu al-Hijjah]], which is also the month of the [[Hajj]].<ref name=GMANewsLeapYearTrivia>{{citation | url = http://www.gmanetwork.com/news/story/249829/lifestyle/people/leap-year-trivia-you-might-want-to-know | title = Leap year trivia you might want to know | date = n.d. | publisher = GMA News | access-date = 29 February 2012 | archive-date = 15 May 2013 | archive-url = https://web.archive.org/web/20130515233835/http://www.gmanetwork.com/news/story/249829/lifestyle/people/leap-year-trivia-you-might-want-to-know | url-status = live }}</ref>

[[Image:Jalaali_Leap_Year.svg]]
{{see also||Jalali calendar }}
The [[Solar Hijri calendar]] is the [[Iranian calendar#Modern calendar (Solar Hejri)|modern Iranian calendar]]. It is an observational calendar that starts on the [[spring equinox (Northern Hemisphere)]] and adds a single intercalated day to the last month (Esfand) once every four or five&nbsp;years; the first leap year occurs as the fifth year of the typical 33-year cycle and the remaining leap years occur every four&nbsp;years through the remainder of the 33-year cycle. This system has less periodic deviation or jitter from its mean year than the Gregorian calendar and operates on the simple rule that New Year's Day must fall in the 24&nbsp;hours of the vernal equinox.<ref name=jitter>{{citation |last=Bromberg |first=Irv |title=Fixed Arithmetic Calendar Cycle Jitter |publisher=University of Toronto |url=http://individual.utoronto.ca/intervalmath/ |access-date=24 October 2019 |archive-date=24 October 2019 |archive-url=https://web.archive.org/web/20191024025932/http://individual.utoronto.ca/intervalmath/ |url-status=live }}</ref> The 33-year period is not completely regular; every so often the 33-year cycle will be broken by a cycle of 29&nbsp;years.<ref>{{citation | first = M. | last = Heydari-Malayeri | url = http://aramis.obspm.fr/~heydari/divers/ir-cal-eng.html | title = A Concise Review of the Iranian Calendar | publisher = Paris Observatory | date = 2004 | arxiv = astro-ph/0409620 | bibcode = 2004astro.ph..9620H | access-date = 19 December 2010 | archive-date = 16 July 2011 | archive-url = https://web.archive.org/web/20110716115426/http://aramis.obspm.fr/~heydari/divers/ir-cal-eng.html | url-status = live }}</ref>

The Hijri-Shamsi calendar, also adopted by the [[Ahmadiyya]] Community, is based on solar calculations and is similar to the Gregorian calendar in its structure with the exception that its [[epoch]] is the [[Hijra (Islam)|Hijra]].<ref>{{citation | url=http://www.alislam.org/library/history/ahmadiyya/73.html | title=Hijri-Shamsi Calendar | publisher=Al Islam | date=2015 | access-date=18 April 2015 | quote=The time frame in these months is the same as [...] the months of a Christian calendar. | archive-date=26 January 2017 | archive-url=https://web.archive.org/web/20170126203234/https://www.alislam.org/library/history/ahmadiyya/73.html | url-status=live }}</ref>

==Coptic and Ethiopian calendars==
The [[Coptic calendar]] has 13&nbsp;months, 12 of 30&nbsp;days each, and one at the end of the year of 5&nbsp;days, or 6&nbsp;days in leap years. The Coptic Leap Year follows the same rules as the Julian Calendar so that the extra month always has 6&nbsp;days in the year before a Julian Leap Year.<ref name = FrTadros>{{citation |author=Fr Tadros Y Malaty |date=1988 |title=The Coptic Calendar and Church of Alexandria. |url=http://www.coptics.info/NEW_ARTICLES2/Others/Z_done/The_Coptic_Calendar_And_The_Church_Of_Alexandria.docx |publisher=The Monastery of St. Macarius Press, The Desert of Scete |access-date=13 September 2022 |archive-date=13 September 2022 |archive-url=https://web.archive.org/web/20220913010235/http://www.coptics.info/NEW_ARTICLES2/Others/Z_done/The_Coptic_Calendar_And_The_Church_Of_Alexandria.docx |url-status=usurped}}</ref> The [[Ethiopian calendar]] has 12&nbsp;months of 30&nbsp;days plus 5 or 6 [[intercalation (timekeeping)|epagomenal days]], which comprise a 13th month.<ref>{{Citation |date=10 September 2021 |title=Ethiopia: The country where a year lasts 13 months |language=en-GB |work=BBC News |url=https://www.bbc.com/news/world-africa-57443424 |access-date=17 September 2022 |archive-date=18 September 2022 |archive-url=https://web.archive.org/web/20220918194518/https://www.bbc.com/news/world-africa-57443424 |url-status=live }}</ref>

==See also==
* [[Century leap year]]
* [[Calendar reform]] includes proposals that have not (yet) been adopted.
* [[Leap second]]
* [[Leap week calendar]]
* [[Leap year bug]]
* [[Sansculottides]]
* [[Zeller's congruence]]
* 30 February
* [[Leap year starting on Monday]]
* [[Leap year starting on Tuesday]]
* [[Leap year starting on Wednesday]]
* [[Leap year starting on Thursday]]
* [[Leap year starting on Friday]]
* [[Leap year starting on Saturday]]
* [[Leap year starting on Sunday]]
*[[Orbital period]]

==Notes==
{{notelist}}

==Sources==
{{refbegin}}
* {{Cite book|title=A Handbook of Dates for students of British History |editor-first= Christopher Robert  |editor-last=Cheney | others= Revised by Michael Jones |year= 2000 |orig-date=1945 |publisher= [[Cambridge University Press]] |location=Cambridge |isbn=9780521778459 |url=https://archive.org/details/cheney-handbook-of-dates/mode/1up}}
* {{cite journal |first=A F |last=Pollard |title=New Year's Day and Leap Year in English History |pages=177{{ndash}}193 |journal=The English Historical Review |volume= 55 |number= 218 (April 1940)|year=1940 |doi=10.1093/ehr/LV.CCXVIII.177 |jstor=553864}}
{{refend}}

==References==
{{Reflist}}

==External links==
* {{cite web|last=Gray|first=Meghan|title=29 Leap Year|url=http://www.numberphile.com/videos/leapyear.html|work=Numberphile|publisher=[[Brady Haran]]|access-date=6 April 2013|archive-url=https://web.archive.org/web/20170522022942/http://www.numberphile.com/videos/leapyear.html|archive-date=22 May 2017|url-status=dead}}
* [https://web.archive.org/web/20170922100012/http://www.leapyearday.com/content/famous-leapers Famous Leapers]
* [https://web.archive.org/web/20100115082230/http://www.thesomnambulist.org/2008/12/galileo-day-campaign-29-february-2012/ Leap Day Campaign: Galileo Day]
* [https://web.archive.org/web/20160227050713/http://news.nationalgeographic.com/2016/02/160226-leap-year-science-time-world-cultures-february/ History Behind Leap Year] [[National Geographic Society]]

{{Time Topics}}
{{Time measurement and standards}}
{{Calendars}}
{{Authority control}}

{{DEFAULTSORT:Leap Year}}
[[Category:Calendars]]
[[Category:Types of year]]
[[Category:Units of time]]