Why February have 29 days in a Leap Year? [video]

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.

Once in four years, February will have an additional 29th day instead of the usual 28, so the year lasts 366 days instead of the usual 365.