Century common year

In the Gregorian calendar, a century common year is a year that is divisible by 100 but not by 400. Therefore, like all other common years, these years do not have a February 29. This is the only case where a year that is divisible by 4 is a common year.

In the obsolete Julian calendar, all years divisible by 4, including century years, were leap years. This rule resulted in too many leap years, so the Gregorian calendar added a new rule that a century year must be divisible by 400 to be a leap year. This is why the years 1600, 2000, and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200, and 2300 are common years even though they are all divisible by 4, as well as 2500, 2600, 2900, and 3000 are also divisible by four as these century common years. The Gregorian calendar was introduced in 1582, so the first century common year was 1700.

The start days of century common years varies. The century year begins on Friday if the remainder obtained by dividing it by 400 is 100, Wednesday if the remainder is 200, and Monday if the remainder is 300. Thus, century leap years always begin on Saturday.