Common year Common year A common year is a calendar year with 365 days. 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 and December 31 fell on a Sunday in 2017) and the year following a common
Leap year leap day "every" four years amounts to about three extra days. The Gregorian calendar therefore drops three leap days every 400 years, which is the length of its "leap cycle". This is done by dropping February 29 in the three century years (multiples of 100) that cannot be exactly divided by 400. The years 1600, 2000 and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200 and 2300 are common years. By this rule, the average number of days per year is 365 + − + = 365.2425. The rule can be applied to years before the Gregorian reform