Leap years are years where an extra, or intercalary, the day is added to the end of the shortest month, February. The February 29, is commonly referred to as leap day. Leap years have 366 days instead of the usual 365 days and occur almost every four years. Leap days keep our modern-day Gregorian calendar in alignment with Earth's revolutions around the Sun. It takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the Sun. This is called a tropical year, and it starts on the March equinox. However, the Gregorian calendar has only 365 days in a year. If we didn't add a leap day on February 29 almost every four years, each calendar year would begin about 6 hours before the Earth completes its revolution around the Sun. Leap days fix that error by giving Earth the additional time it needs to complete a full circle around the Sun.