The Earth rotates about 365.242375 times each year, even though a normal year is only 365 days. To accommodate the extra 0.242375 days each year, every four years we have a "leap year" that includes an extra day. A leap year, therefore, has 366 days rather than 365. The extra day is added in February; every four years, we have a February 29.

Every Four Years ...

Generally, leap years happen every four years. More specifically, they occur in any year that can be evenly divided by the numeral 4. For example, we had a leap year in 2012, as 2012 is divisible by 4. 2016 is also divisible by 4, so 2016 will be a leap year too. After 2016, upcoming leap years will be 2020, 2024, 2028 and so on, but 2100 is an exception to this rule.

Dividing by 100

If a year can be divided evenly by 100, it's not considered a leap year. The year 2100, for example, won't "leap." This is because every 100 years, the Earth’s rotations do not add up to an extra day: .24 extra days per year multiplied by 100 years leaves you with only .0024 to make up, which is not a full day. But if the year can be divided evenly by 400, it will be a leap year. For example, 2000 was a leap year, even though it was divisible by 100, because in the Earth’s rotation, we gain one day every 400 years, or 0.0025 days per year, and we need to add that extra day to the calendar.