Friday is the bonus day in 2008, the extra day, leap day. If you check on Wikipedia you will find a very succinct description covering several cultures telling you that the calendar is a complicated thing. If you want the backstory, perhaps a more detailed and readable explanation of how leap day came to be, check on our shelves for the history of the calendar. It involves math, history, religion, astronomy, personalities and several continents. Put those extra 24 hours to use and learn something new!