A leap year comes once every four years. Because of this, a leap year can always be evenly divided by four. For example, 2020 was a leap year. But a year is a common year if it can be evenly divided by 100 but not by 400. This is why 1600, 2000, and 2400 are leap years, but not 1700, 1800, 1900, 2100, 2200, or 2300.