A leap year has 366 days. This is one day more than a common year, which has 365 days. The extra day is added to the month of February, making it 29 days long instead of the usual 28. Leap years occur approximately every four years to keep our calendar synchronized with the Earth's orbit around the Sun, which takes about 365.25 days. The leap year rule is that the year must be divisible by 4, except for years divisible by 100 unless they are also divisible by 400. Hence, 2020 was a leap year, 1900 was not, and 2000 was.