A leap year has 366 days instead of 365 days. The addition of this extra day to the Gregorian calendar occurs in the month of February.
In a leap year, Feb. 29 is the extra day of the year, which is called either a leap day or an intercalary day. Leap years occur approximately every four years. The addition of an extra day to the Gregorian calendar occurs so that it aligns and correctly matches the length of a solar year. A solar year is the time it takes for the Earth to make one revolution around the sun. It is approximately equal to 365.242199 days. Without this correction, a calendar year is shorter than a solar year.