A normal calendar year has 365 days. However, Earth needs about 365.2422 days to orbit the Sun. This small difference is almost six hours each year.
The extra time does not disappear. After about four years, it becomes almost one full day. We add February 29 to the calendar, and that year has 366 days. It is called a leap year.
Earth’s year is not exactly 365.25 days, so there is another rule. Years divided by 100 are usually not leap years, unless they can also be divided by 400. For example, 2000 was a leap year, but 1900 was not. These rules help keep our calendar close to the seasons.