A leap year occurs every four years. It is a year with an extra day. We have leap years because there are 365.25 days in a year so the years will be even.
There are 365.24 or 365 and 1/4 days in a year, so when 4 years come together, we get an extra day, that year is known as Leap year.
A leap year is a year with an extra day—February 29. Leap years occur on years that are exactly divisible by four (with no remainder), with the exception of most century (also called "centenary") years. A century year is a year that ends in the digits "00." Only century years divisible by 400 are leap years. For instance, even though 1900 is divisible by four (1900/4 equals 475), 1900 was not a leap year. That's because 1900 is not perfectly divisible by 400 (1900/400 equals 4.75) The next century year, 2000, will be a leap year.
Source: Burnam, Tom. The Dictionary of Misinformation, p. 136.