What is a leap year?
Every four years, an extra day is added to our calendar year, making it 366 days long rather than the usual 365. This additional day comes at the end of February, therefore February 29th is often referred to as "leap day."
The origins of leap year date far back in history:
In 45 BC Julius Caesar…decided to use a purely solar calendar. This calendar, known as the Julian calendar, fixed the normal year at 365 days, and the leap year, every fourth year, at 366 days. Leap year is so named because the extra day causes any date after February in a leap year to 'leap' over one day in the week and to occur two days later in the week than it did in the previous year, rather than just one day later as in a normal year. (from MSN Encarta)
Part of the purpose of a leap year is to keep our calendar in alignment with the earth's movement around the sun. For more information about this and other leap year facts, check out timeanddate.com.