Why do we have leap years?

Calendars are fairly fixed things. Each year has the same number of months and days, and the days follow a seven-day rotation going from Sunday to Saturday. But the movements of Earth do not conform exactly to the time designations humans have imposed.

For example, the calendar used in much of the world, the Gregorian calendar, says that a year has 365 days. In fact, it takes Earth about 365.25 days to go around the Sun. That extra one-quarter day must be accounted for in the calendar or, eventually, the calendar and the seasons of the year would no longer be aligned. To correct this problem, an extra day is added to the calendar every four years. In such years, called leap years, February has one extra day, or 29 days. When the Gregorian calendar was devised, astronomers realized that even adjusting the calendar to add one day every four years would still not make it match exactly the movements of Earth. So they decreed that when that fourth year falls in a century year (one with two zeros at the end) that is not divisible by 400 (like 1700 or 1900) there would not be an extra day.