How often does "Leap Year" happen?

A leap year happens every four years in the Gregorian calendar, with one extra day added to February.

That extra day, February 29, keeps the calendar aligned with Earth’s orbit around the Sun. A standard calendar year has 365 days, but the tropical year is about 365.2422 days long. Without periodic correction, calendar dates would gradually drift away from the seasons.

The rule has an important exception. Years divisible by 100 are not leap years unless they are also divisible by 400. Thus 2000 was a leap year, while 1700, 1800, and 1900 were not. This means the precise rule is “almost every four years,” rather than literally every four years.

The idea goes back to the Julian calendar introduced under Julius Caesar in 46 BCE. The Gregorian calendar, adopted in 1582 and now used by most countries, refined the system by removing three leap days every 400 years.