The earth takes 365¼ days (one year ) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for convenience. These six hours saved every year are added to make one day i.e. 24 hours over a span of four years. This surplus day is added to the month of February. Thus, every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.