A common year is a year that is not a leap year. In the Gregorian calendar a common year has 365 days. This means a common year has 52 weeks and one day. So if a certain year started on a Monday, the following year will start on a Tuesday. Stated differently, a common year always begins and ends on the same day of the week. (For example, in 2009, both January 1 and December 31 fell on a Thursday.)