A century leap year is a leap year in the Gregorian calendar that is evenly divisible by 400.^[1]

Like all leap years, it has an extra day in February for a total of 366 days instead of 365. In the obsolete Julian calendar, all years that were divisible by 4, including end-of-century years, were considered leap years. The Julian rule, however, adds too many leap days (about 3 extra leap days in 400 years), which resulted in the calendar drifting gradually with respect to the astronomical seasons. To remedy this, Pope Gregory XIII introduced in 1582 a slightly modified version of the Julian calendar, the Gregorian calendar, where century years are leap years only if they are divisible by 400. This eliminates 3 of the 4 end-of-century years in a 400-year period. For example, the years 1600, 2000, 2400, and 2800 are century leap years since those numbers are evenly divisible by 400, while 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 are common years despite being evenly divisible by 4. This scheme brings the average length of the calendar year significantly closer to the astronomical length of the year, nearly eliminating the drift of the calendar against the seasons.

The Gregorian calendar was adopted by various countries at different times over several centuries. Dates prior to 1582 are generally recorded using the Julian calendar, and different countries have different conventions about how to record dates between 1582 and their adoption of the Gregorian calendar.^[a] Consequently, for example, the year 1700 was a leap year in the British and Russian empires but not in most of the rest of Europe; 1800 and 1900 were still leap years in the Russian empire but not generally elsewhere.^[b]

• An Introduction to Calendars courtesy of the United States Naval Observatory Archived 2019-06-13 at the Wayback Machine
• Frequently Asked Questions about Calendars
• History of Gregorian Calendar