Common year

A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366. More generally, a common year is one without intercalation. The Gregorian calendar (like the earlier Julian calendar) employs both common years and leap years to keep the calendar aligned with the tropical year, which does not contain an exact number of days.
The common year of 365 days 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 fell on a Friday and December 31 will fall on a Friday in 2021, the current year) and the year following a common year will start on the subsequent day of the week. In common years, February has exactly four weeks, so March begins on the same day of the week. November also begins on this day.
In the Gregorian calendar, 303 of every 400 years are common years. By comparison, in the Julian calendar, 300 out of every 400 years are common years, and in the Revised Julian calendar (used by Greece) 682 out of every 900 years are common years.