A leap year is a year that has 366 days instead of the usual 365 days. This is done to keep the calendar year synchronized with the astronomical year, which is the time it takes Earth to orbit the Sun. The astronomical year is about 365.2422 days long, so a leap year is added every four years to make up for the extra time.

The rules for leap years are as follows:

• A year is a leap year if it is divisible by 4.
• However, a year that is divisible by 100 is not a leap year unless it is also divisible by 400.

This means that the years 2000, 2004, 2008, 2012, 2016, 2020, 2024, and so on are leap years, but the years 1700, 1800, 1900, 2100, 2200, 2300, and 2500 are not leap years.

The extra day in a leap year is added to February, which becomes 29 days long instead of 28 days. This is why February 29 is sometimes called "leap day".

Leap years are necessary to keep the calendar year synchronized with the astronomical year. Without leap years, the seasons would slowly drift out of sync with the calendar. This would eventually cause problems for farmers and other people who rely on the seasons.