What is a leap year?

Which years are leap years?

In the Gregorian calendar, the calendar used by most modern countries, the following three criteria determine which years will be leap years:

1. Every year that is divisible by four is a leap year;
2. of those years, if it can be divided by 100, it is NOT a leap year, unless
3. the year is divisible by 400. Then it is a leap year.