Why do we have leap years?

A calendar year of 365 days is almost six hours shorter than the solar year, so without correction the seasons would drift by about a day every four years. Julius Caesar introduced a leap day every fourth year in 45 BC, but this overcorrected slightly because the true excess is a little under six hours. By the 1500s the calendar had drifted about 10 days, so Pope Gregory XIII introduced the Gregorian calendar in 1582. Under its rule, century years are leap years only if divisible by 400, so 2000 was a leap year but 1900 was not. This system keeps the calendar accurate to within about one day every 3,000 years.