The movement of the Earth on its axis around the sun takes 365 days (one year) and 6 hours to revolve once around the sun. We ignore six hours for the sake of convenience and consider a year consisting of 365 days. Saved six hours are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February and in every fourth year, February is of 29 days in spite of 28 days.