It takes the earth about 365.2422 days to go around the sun, but a normal calendar year is only 365 days. The extra fraction of a day adds up: circling the sun four times takes 1460.9688 days, but four calendar years would only be 1460 days. That .9688 is almost a whole day, so every four years we add an extra day to our calendar, February 29. We call that year leap year. To make things easier, leap years are always divisible by four: 2004 and 2008 will both be leap years.
For hundreds of years, people used a calendar called the Julian calendar that followed this rule, adding a leap year every four years. However, because .9688 isn’t exactly a whole day, the Julian calendar slowly began to disagree with the real seasons. In 1582, Pope Gregory fixed this problem by ordering everyone to use a new set of rules. These rules are named the Gregorian calendar, after him. They work like this:
The Gregorian Calendar
Every fourth year is a leap year. 2008, 2012, and 2016 are leap years.
However, every hundredth year is not a leap year. 1900 and 2100 are not leap years.
Every four hundred years, there’s a leap year after all. 2000 and 2400 are leap years.
People in English-speaking countries didn’t start using the Gregorian calendar until 1752. Some countries, such as Iran, still use other systems.