We have leap years because instead of 365 days, the Earth really takes a few minutes less than 365-1/4 days (365.24219) to go completely around the Sun. Without leap years, the seasons would start one day earlier on the calendar every four years. After 360 years, spring in the Northern Hemisphere and autumn in the Southern Hemisphere would begin on December 21 (which is when winter in the Northern Hemisphere and summer in the Southern Hemisphere presently begins).