Century 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. The Gregorian calendar was introduced in 1582, so the first century common year was 1700.