Why does a leap year have 366 days?science questions for kids Fascinating facts about the World.Actually, A common year has 365 days and a leap year 366 days, with the extra day designated as February 29. A leap year occurs every four years to help synchronize the calendar year with the solar year, or the length of time it takes the earth to complete its orbit about the sun,Watch the video to know more about leap year. https://www.youtube.com/watch?v=GwGurhXS1aQ