This year is a leap year and February 29 is the leap day. It happens every 4 years in the Gregorian calendar used by most of the world. Leap years have 366 calendar days instead of the normal 365 in order to keep in sync with earth's journey around the sun.
When Pope Gregory XIII added the leap day to the Gregorian calendar in 1582, he chose February because it was the shortest month, making it one day longer on leap years.
For math gurus: The system is not perfect: We gain around 44 extra minutes every four years, or a day every 129 years. To solve this problem, we skip the leap years every centenary year except for those that are divisible by 400, such as 1600 and 2000.