a leap year is a year in which an extra day is added to the gregorian calendar, which is used by most of the world. a common year has 365 days, but a leap year has 366 days. the extra day, february 29, is added to the month of february. in a common year, february has 28 days, but in a leap year it has 29 days. the extra day, called a leap day, occurs on the same day of the week as the first day of the month, february 1.