<P> A leap year (also known as an intercalary year or bissextile year) is a calendar year containing one additional day (or, in the case of lunisolar calendars, a month) added to keep the calendar year synchronized with the astronomical or seasonal year . 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 drift over time with respect to the event that the year is supposed to track . By inserting (also called 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 . </P> <P> For example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28 . These extra days occur in years which are multiples of four (with the exception of years divisible by 100 but not by 400). 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 . In the Bahá'í Calendar, a leap day is added when needed to ensure that the following year begins on the vernal equinox . </P>

What is the reason behind the leap year