Today is Leap Day! February 29th is a date that usually occurs every four years, and is called Leap Day. This day is added to the calendar in leap years as a corrective measure, because the earth does not orbit around the sun in precisely 365 days.
In the Gregorian calendar, the current standard calendar in most of the world, most years that are evenly divisible by four are leap years. In each leap year, the month of February has 29 days instead of 28. Adding an extra day to the calendar every four years compensates for the fact that a period of 365 days is shorter than a solar year by almost six hours.