Insider Blog: Leap years explained

We're all taught that there are 365 days in a year. But, once every four years we add one extra day. This is because we use a solar calendar. One year is technically the time it takes for the sun to orbit the sun. One orbit comes out to 365.25 days. As a result, we include February 29th every 4th year to account for the extra time.

Earth isn't the only planet that experiences leap years. On Mars, you would need to add an extra day, or sol as they're called on the red planet, 6 out of every 10 years.