Leap year has one more February 29th than normal year, which is often called "leap day". Except February, the days of other 12 months have not changed. In order to make the rules of leap year, people have made the following rules: leap every four years, but not every one hundred years, but every four hundred years. For example, 2000 is a leap year, but 1900 is not a leap year, because it is a century year (that is, it ends with 00) but it is not a year divisible by 400.
The history of leap years can be traced back to ancient Rome, when people made calendars according to the cycles of the moon and the sun. At that time, the calendar was only 10 months, and there were only 304 days in a year, less than a year. Later, in order to solve this problem, King Julius Caesar of Rome adopted the advice of Egyptian astronomers and divided the year into 365 days, plus a leap day, that is, a leap year of 366 days. This provision has been used to this day, becoming the ancestor of the modern public calendar, also known as the "Gregory calendar".