A common year is a calendar year with exactly 365 days, in contrast to the longer leap year. 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 adjust for differing astronomical measurements of the year: sidereal and tropical.
The common year of 365 days has exactly 52 weeks and one day, hence a common year always begins and ends on the same day of the week. (For example: both January 1 and December 31 fell on a Friday in 2010). In a common year, February has exactly four weeks, so that month and March always start consecutively on the same day of the week.
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.
In the Lunisolar calendar and the Lunar calendar, a common year consists of 354 days.