<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>
what is the reason behind the leap year
to keep the calendar year synchronized with the astronomical or seasonal year