Leap year

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. Since astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days each year will unavoidably drift over time with respect to the seasons. By inserting an additional day—a leap day—or an additional month—a leap month—into some years but not others, the drift between a civilisation's dating system and the physical properties of the Solar System can be avoided.

An astronomical year lasts slightly less than 3651⁄4 days. The historic Julian calendar has three common years of 365 days followed by a leap year of 366 days, by extending February to 29 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; 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. Thus 1600, 2000 and 2400 are leap years, but 1700, 1800, 1900, 2100, 2200, and 2300 are not. In the Solar Hijri and Bahá'í calendars, a leap day is added whenever needed to ensure that the following year begins on the March equinox.