Common year

A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366 days.^[1] More generally, a common year is one without intercalation. The Gregorian calendar, used by the majority of the world, employs both common years and leap years. This is to keep the calendar aligned with the tropical year, which does not contain an exact number of days. A common year is approximately a quarter day (six hours) shorter than a tropical year, which has 365.24 days.^[2] If the Gregorian calendar only used common years and omitted leap years, the calendar would be out of sync with the tropical year by approximately 24 days in 100 years.^[3]

In the Gregorian calendar, 303 out of every 400 years are common years. Leap years are any years that are divisible by 4, unless it can also be divided by 100, in which case it is a common year. One exception is if the year can be divided by 4, 100, and 400 - these years are leap years. The extra common years are added to account for the fact that common years are 5 hours, 48 minutes, and 46 seconds shorter than a tropical year, rather than six hours exactly.^[3] By comparison, in the Julian calendar, 300 out of every 400 years are common years, with every fourth year being a leap year without exception.

The common year has 52 weeks and one day, hence a common year always begins and ends on the same day of the week (for example, January 1 and December 31 both fall on a Wednesday in 2025) and the year following a common year will start on the subsequent day of the week. As a result, if the following year is a common year as well, each date will advance by one day of the week. For example, June 1, 2025 falls on a Sunday, so it falls on a Monday in 2026 and Tuesday in 2027. If the following year is a leap year, then dates from January 1 to February 28 will still advance by one day, but all subsequent dates will advance by two days (for example, March 1 falls on a Monday in 2027 and Wednesday in 2028), due to the additional day.^[4]