Leap year

Calendar year with a day (or month) added

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.

From Wikipedia, under CC BY-SA. More on occurri.