Today, Feb. 29, is a day that only comes around once every four years.
The reason – we must add an additional day to the calendar every four years because of the time it takes the Earth to revolve around the sun.
It takes our planet about 365 ¼ days, roughly, to make a complete revolution around the sun. Our calendar, however, only has 365 days each year. If we didn't add an extra day every four years, our calender would be off by about 24 days in 100 years.