 What is a leap year? A leap year is a calendar year that has an extra day, making a leap year 366 days long, instead of 365. The extra day is added to February, making February 29th, leap day. You see, the Gregorian calendar has some rules. First, if a year is divisible by four, it's a leap year. But if it's divisible by a hundred, and also divisible by four, it's not a leap year. However, if it's divisible by 400, it's back to be a leap year. Will the year 3000 be a leap year?