How often does leap year occur?

A leap year has 366 days, as opposed to a common year, which has 365. Nearly every 4 years is a Leap Year, and we add a Leap Day, an extra – or intercalary – day on February 29. Leap Years are needed to keep our modern day Gregorian Calendar in alignment with the Earth's revolutions around the sun. It takes the Earth approximately 365.242199 days – or 365 days, 5 hours, 48 minutes, and 46 seconds – to circle once around the Sun. This is called a tropical year.

However, the Gregorian calendar has only 365 days in a year, so if we didn't add a day on February 29 nearly every 4 years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by approximately 24 days!