<P> The name "leap year" probably comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, the day of the week in the 12 months following the leap day (from March 1 through February 28 of the following year) will advance two days due to the extra day (thus "leaping over" one of the days in the week). For example, Christmas Day (December 25) fell on a Monday in 2017, then it will fall on Tuesday in 2018, and Wednesday in 2019 but then "leaps" over Thursday to fall on a Friday in 2020 . </P> <P> The length of a day is also occasionally changed by the insertion of leap seconds into Coordinated Universal Time (UTC), owing to the variability of Earth's rotational period . Unlike leap days, leap seconds are not introduced on a regular schedule, since the variability in the length of the day is not entirely predictable . </P> <P> In the Gregorian calendar, the standard calendar in most of the world, most years that are multiples of 4 are leap years . In each leap year, the month of February has 29 days instead of 28 . Adding one extra day in the calendar every four years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours . Some exceptions to this basic rule are required since the duration of a tropical year is slightly less than 365.25 days . The Gregorian reform modified the Julian calendar's scheme of leap years as follows: </P> <P> Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400 . For example, the years 1700, 1800, and 1900 were not leap years, but the years 1600 and 2000 were . </P>

Why do we have to have leap years
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