February 29 is a date that occurs only in leap years, which are years that have an extra day added to the month of February. This day is included to help synchronize the calendar year with the solar year, as it takes approximately 365.25 days for the Earth to orbit the Sun. Leap years happen every four years, except for years that are divisible by 100 but not by 400. The concept of leap years was introduced by the ancient Romans and is now part of the Gregorian calendar, which is the calendar system used by most of the world today. People born on February 29 are often called "leap year babies" and may celebrate their birthdays on February 28 or March 1 in non-leap years.