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 2021 (the previous year), both January 1 and December 31 falls on a Friday.)
In the Gregorian calendar, 303 out of every 400 years are common years. In the Julian calendar, 300 out of every 400 years were common years. All the other years are special and known as leap years.