This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The Gregorian calendar has 97 leap years every 400 years:
Every year divisible by 4 is a leap year.
However, every year divisible by 100 is not a leap year.
However, every year divisible by 400 is a leap year after all.
So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
2000, and 2400 are leap years.
(Destruction of a myth: There are no double leap years, i.e. no years
with 367 days. See, however, the note on Sweden in section 2.2.4.)