Complete list of leap years from 1900 to 2100. Leap years happen every 4 years, except for century years not divisible by 400. 49 leap years in this period.
A year is a leap year if it is divisible by 4 — except if it is divisible by 100 but not by 400.
So 2000 was a leap year (divisible by 400), 1900 was not (divisible by 100 but not by 400), and 2100 will not be.
This rule keeps the calendar year synchronized with the astronomical year of approximately 365.2425 days.
In a leap year, February has 29 days instead of 28, making the total year 366 days.