A common year is a calendar year that has exactly 365 days, as opposed to a leap year which has 366 days. This matters because most years are common years, and understanding the difference helps explain why we need leap years periodically to keep our calendar aligned with Earth's orbit around the sun.
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A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366 days. 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. 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.
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. 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.
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