Leap years have an extra day to keep the calendar in sync with Earth's orbit around the sun. NASA explains this well. Simply put, a leap year means 366 days instead of the usual 365. Leap Day, February 29th, is the extra day tacked on at the end of February. These years pop up roughly every four years. Why do we need them? Earth's journey around the sun takes about 365.25 days. By adding a day every four years, we make sure seasons stay consistent and don't shift over time.