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Snippet from Wikipedia: 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.

A lunar year of 12 lunar months is about 11 days shorter than an astronomical year. Lunisolar calendars, such as are traditional in most of Asia, each has its own rule to decide when to add a leap (lunar) month, so as to keep its calendar year from drifting through the seasons. Pure lunar calendars, such as the lunar Hijri calendar used in most of Islam, do not employ any such arrangement and accept the seasonal drift.

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 months following the leap day (from 1 March through 28 February of the following year) will advance two days due to the extra day, thus leaping over one day in the week. For example, since 1 March was a Friday in 2024, a Saturday in 2025, will be a Sunday in 2026, and a Monday in 2027, the date will then "leap" over Tuesday to fall on a Wednesday in 2028.

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.

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