A leap year (or intercalary year) is a year containing one or more extra days (or, in case of lunisolar calendars, an extra month) in order to keep the calendar year synchronised with the astronomical or seasonal year. For example, February would have 29 days in a leap year instead of the usual 28. Seasons and astronomical events do not repeat at an exact number of full days, so a calendar which had the same number of days in each year would over time drift with respect to the event it was supposed to track. By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected. A year which is not a leap year is called a common year.

The Gregorian calendar, the current standard calendar in most of the world, adds a 29th day to February, 97 years out of 400, a closer approximation than once every four years. This would be implemented by making a leap year every year divisible by 4 unless that year is divisible by 100. If it is divisible by 100 it would only be a leap year if that year was also divisible by 400.[1][2] So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300, 2500, 2600, 2700, 2900 and 3000 will not be leap years, but 2400 and 2800 will be. The years that are divisible by 100 but not 400 are known as "exceptional common years". By this rule, the average number of days per year will be 365 + 1/4 - 1/100 + 1/400 = 365.2425. The Gregorian calendar was designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox.[3] Therefore, with the vernal equinox year about 365.242375 days long, the average year length of 365.2425 days made by the Gregorian calendar is a good enough approximation. This marginal difference of 0.000125 days means that in around 8,000 years, the calendar will be about one day behind where it should be. But in 8,000 years, the length of the vernal equinox year will have changed by an amount which can not be accurately predicted (see below). Therefore, the current Gregorian calendar suffices for practical purposes.