Leap year: Encyclopedia II - Leap year - Gregorian calendar

Leap year - 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 calendar is designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox.
• The vernal equinox year is currently about 365.242375 days long.
• 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.

Leap year - 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 days.

Adapted from the Wikipedia article "Gregorian calendar", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki