"Leap years are years with 366 calendar days instead of the normal 365. They happen every fourth year in the Gregorian calendar — the calendar used by the majority of the world. The extra day, known as a leap day, is Feb. 29, which does not exist in non-leap years. Every year that is divisible by four, such as 2020 and 2024, is a leap year except for some centenary years, or years that end in 00, such as 1900.” - Live Science