The Earth takes exactly 365.24219 days to rotate around the sun. Adding an extra day every four years makes the average length of a year 365.25 days, slightly too much. The Gregorian calendar eliminated leap days in years ending in 00 and that made the average year 365.24 days, a bit too short. The solution: add a leap day in years divisible by 400 which makes the average year 365.2425.