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==Gregorian calendar==
The [[Gregorian calendar]], the current standard calendar in most of the world, adds a 29th day to [[February]] in all years evenly divisible by 4, except for century years (those ending in -00), which receive the extra day only if they are evenly divisible by 400. Thus 1996 was a leap year whereas 1999 was not, and 1600, 2000 and 2400 are leap years but 1700, 1800, 1900 and 2100 are not.
 
The reasoning behind this rule is as follows:
*The Gregorian leap year rule gives an average year length of 365.2425 days.
 
This difference of a little over 0.0001 days means that in around 8,000 years, the calendar will be about one day behind where it should be. But in 8,000 years' time the length of the vernal equinox year will have changed by an amount we can't accurately predict (see below). So the Gregorian leap year rule does a good enough job.
 
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===Which day is the leap day?===
The Gregorian calendar is a modification of the Julian calendar first used by the Romans. The [[Roman calendar]] originated as a [[lunar calendar]] (though from the [[5th century BC]] it no longer followed the real moon) and named its days after three of the phases of the moon: the new moon (''calends'', hence "calendar"), the first quarter (''nones'') and the full moon (''ides''). Days were counted down (inclusively) to the next named day, so [[24 February]] was ''ante diem sextum calendas martii'' ("the sixth day before the calends of March").
 
Since [[45 BC]], February in a leap year had ''two'' days called "the sixth day before the calends of March". The extra day was originally the second of these, but since the [[third century]] it was the first. Hence the term '''bissextile day''' for [[24 February]] in a '''bissextile year'''.
 
Where this custom is followed, anniversaries after the inserted day are moved in leap years. For example, the former feast day of [[Saint Matthias]], [[24 February]] in ordinary years, would be [[25 February]] in leap years.
 
This historical nicety is, however, in the process of being discarded: The [[European Union]] declared that, starting in 2000, [[29 February]] rather than [[24 February]] would be leap day, and the [[Roman Catholic Church]] also now uses [[29 February]] as leap day. The only tangible difference is felt in countries that celebrate [[feast day]]s.
 
==Julian calendar==
The [[Julian calendar]] adds an extra day to February in years divisible by 4.
 
This rule gives an average year length of 365.25 days. The excess of about 0.0076 days with respect to the [[vernal equinox year]] means that the vernal equinox moves a day earlier in the calendar every 130 years or so.
 
==Revised Julian Calendar==
The [[Revised Julian calendar]] adds an extra day to February in years divisible by 4, except for years divisible by 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 the 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… days. This is a very good approximation to the ''mean'' [[tropical year]], but because the ''vernal equinox'' tropical year is slightly longer, the Revised Julian calendar does not do as good a job as the Gregorian calendar of keeping the vernal equinox on or close to [[21 March]].
 
==Chinese calendar==
 
==Hebrew calendar==
The Hebrew calendar is also [[lunisolar calendar|lunisolar]] with an embolistic month. In the [[Hebrew calendar]] the extra month is called ''Adar Alef'' ([[Adar_1|first Adar]]) and is added before ''[[Adar]]'', which then becomes Adar Sheni ([[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.
 
In addition, the Hebrew calendar has postponement rules that postpone the start of the year by one or two days. The year before the postponement gets one or two extra days, and the year whose start is postponed loses one or two days. These postponement rules reduce the number of different combinations of year length and starting day of the week from 28 to 14, and regulate the location of certain religious holidays in relation to the Sabbath.
 
==Long term leap year rules==
The accumulated difference between the Gregorian calendar and the vernal equinoctial year amounts to 1 day in about 8,000 years. This suggests that the calendar needs to be improved by another refinement to the leap year rule: perhaps by avoiding leap years in years divisible by 8,000.
 
(The most common such proposal is to avoid leap years in years divisible by 4,000 [http://www.google.com/search?q=%22gregorian+calendar%22+error+%22leap+year%22+4000]. This is based on the difference between the Gregorian calendar and the ''mean tropical year''. Others claim, erroneously, that the Gregorian calendar itself already contains a refinement of this kind [http://www.straightdope.com/mailbag/mleapyr.html].)
 
However, there is little point in planning a calendar so far ahead because over a timescale of tens of thousands of years the number of days in a year will change for a number of reasons, most notably:
 
==Marriage proposal==
There is a [[tradition]], said to go back to [[Saint Patrick]] and [[Brigid of Ireland|Saint Bridget]] in [[5th century]] [[Ireland]], whereby women may only make marriage proposals in leap years.
 
===Saint Patrick and the leap year===
(Source: Evans, Ivor H, ''Brewer's Dictionary of Phrase and Fable'', Cassell, London, 1988)
 
According to a [[1288]] law in [[Scotland]], fines were levied if the proposal was refused by the man; compensation ranged from a kiss to a silk gown to soften the blow. Because men felt that put them at too great a risk, the tradition was in some places tightened to restricting female proposals to [[29 February]].
 
==Birthdays==
 
A person who was born on [[29 February]] may be called a "[[leapling]]". In non-leap years they usually celebrate their birthday on [[28 February]] or [[1 March]].
 
There are many instances in children's literature where a person's claim to be only a quarter of their actual age turns out be based on counting their leap-year birthdays. A similar device is used in the plot of the [[Gilbert and Sullivan]] [[operetta]] ''[[The Pirates of Penzance]]''.
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