For example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28. These extra days occur in each year which is an integer multiple of 4 (except for years evenly divisible by 100, which are not leap years unless evenly divisible by 400). The leap year of 366 days has 52 weeks and two days, hence the year following a leap year will start later by two days of the week.