 a leap year (or intercalary or bissextile year) is a year containing one additional day (or, in the case of lunisolar calendars, a month) in order to keep the calendar year synchronized with the astronomical or seasonal year.meeus, jean. (1998.) astronomical algorithims. willmann-bell. pp. 62. because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year, over time, drift with respect to the event that the year is supposed to track. by inserting (or intercalating) an additional day or month into the year, the drift can be corrected. a year that is not a leap year is called a common year. for example, in the gregorian calendar, each leap year lasts 366 days instead of the usual 365, by extending february to 29 days rather than the common 28 days. similarly, in the lunisolar hebrew calendar, adar aleph, a 13th lunar month is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. the name "leap" year 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, in a leap year the day of the week will advance two days (from march onwards) due to the year's extra day inserted at the end of february (thus "leaping over" one of the days in the week). for example, christmas day fell on tuesday in 2001, wednesday in 2002 and thursday in 2003 but then "leapt" over friday to fall on a saturday in 2004.douglas harper, "leap year", online etymology dictionary, 2012. ==gregorian calendar== in the gregorian calendar, the current standard calendar in most of the world, most years that are integer multiples of 4 are leap years. in each leap year, the month of february has 29 days instead of 28. adding an extra day to the calendar every four years compensates for the fact that a period of 365 days is shorter than a solar year by almost 6 hours. this calendar was first used in 1582. some exceptions to this basic rule are required since the duration of a solar year is slightly less than 365.25 days. over a period of 4 centuries, the accumulated error of adding a leap day every 4 years amounts to about 3 extra days. the gregorian calendar therefore omits 3 leap days every 400 years, which is the length of its leap cycle. this is done by omitting february 29 in the 3 century years (integer multiples of 100) that are not also integer multiples of 400.united states naval observatory. (june 14, 2011). leap years. author. for example, 1700, 1800, 1900, 2100 and 2200 are common ye
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