What is the answer for why you have leap year?

The time it takes Earth to orbit the sun is about 365.2422 days. Since calendars consist of only whole days, but 365.2422 is not a whole number, some years need to be shorter than 365.2422 days and some years have to be longer than 365.2422 days at a ratio that makes the average length of a calendar year about 365.2422 days.

The average length of a year of the Julian calendar is 365.25 days, which is accurate to within one day every 128 years. The average length of a year of the Gregorian calendar is 365.2425 days, which is accurate to within one day every 3200 years (25 times more accurate).