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 (an ordinary year) has 365 days, but a leap year has 366 days. The extra day, February 29, is added to the month of February. In an ordinary year, February has 28 days, but in a leap year it has 29 days. The extra day, called a leap day, is the same day of the week as the first day of the month, February 1. Also, in a leap year, the months of January, April, and July all start on the same day of the week.