There are several calendars in use today which apply different rules to determine how often a leap year occurs. Some even add a leap month instead of a leap day.
The Gregorian calendar, used by most western countries, adds one extra day, leap day on February 29, nearly every four years.
When Is the Next Leap Year?
However, some cultures use calendars that do not apply the same leap year rules as the Gregorian calendar.
The Iranian calendar, for instance, doesn't observe February 29 as a leap day. Other calendars, such the Chinese calendar, add a leap month instead of a leap day.
Leap Years in Different Calendars