Which years are leap years? When was the first leap year?

To be a leap year, the year number must be divisible by four – except for end-of-century years, which must be divisible by 400. (So the year 2000 was a leap year, although 1900 was not.)

The first leap year

The first leap year in the modern sense was 1752, when 11 days were 'lost' from the month September with the adoption of the Gregorian calendar by Britain and her colonies. After 1752 we adopted the system still in use today where an additional day is inserted in February in years wholly divisible by four, other than years ending in 00 with the exception of those divisible by 400 which are still leap years (like 2000). This is certainly not the first use of leap years; the Julian calendar we used before 1752 had a simpler system of leap years, and remember, no calendar is universal.