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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
===Which day is the leap day?===
The Gregorian calendar is a modification of the Julian calendar first used by the Romans.
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.
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 [[Julian calendar]] adds an extra day to February in years divisible by 4.
This rule gives an average year length of 365.25 days.
==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.
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]]).
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.
(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].
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:
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 [] 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.
A person who was born on [[29 February]] may be called a "[[leapling]]".
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.
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