Today is Feb. 29...a "leap day." This is a day added to the calendar every few years to make up for the slight inaccuracies of matching up the sun, moon, stars and the rotation of the Earth with the calandar system we use.
I picked up this definition from Wikipedia:
Although most years of the modern calendar have 365 days, a complete revolution around the sun takes approximately 365 days and 6 hours. Every four years, during which an extra 24 hours have accumulated, one extra day is added to keep the count coordinated with the sun's apparent position.