A normal year has 365 days, which is about the time it takes for Earth to orbit the Sun once. However, it actually takes Earth about 365.24 days—roughly 365 and a quarter days—to complete its journey around the Sun.

If we didn't add an extra day every four years, our calendar would slowly fall out of sync with the seasons. After a hundred years, summer would start almost a month later than it should!

To keep our calendar matched with the seasons, we add an extra day to February every four years, creating a 366-day year called a leap year. This makes sure our seasons always happen at the right time of the year.