Common year
In the Gregorian Calendar, which was introduced in 1582 by Pope Gregory XIII, because the Julian calendar added slightly too many years, three leap years were removed for every 400 years. These are those that are multiples of 100, but not multiples of 400. Thus, in the Gregorian calendar, 2020 is a leap year, but not 2019 and 2021, 1900 is not a leap year even though 1904 and 1896 are, and 2000 is a leap year even though 1900 and 2100 are not.