Leap Year is established to make up for the time difference between the number of days in a year and the actual revolution period of the earth caused by the artificial calendar. The year that makes up for the time difference is a leap year. There are 366 days in a leap year (January to December are 31 days, 29 days, 31 days, 30 days, 31 days, 30 days, 31 days, 31 days, 30 days, 31 days, 30 days, 31 days, 30 days, 31 days).