Every 2nd February is a special day in Punxsautawney, Pennsylvania. According to folklore, if it is cloudy when a grounhog emerges from its burrow on this day. If he leaves the burrow, it means that winter-like weather will soon end. If it is sunny, the groundhog will supposedly see its shadow and retreat back into its burrow, and the winter weather will continue for six more weeks.This year is the 125th annual forecast for Phil.

You can find more information in the official web: Groundhog Day official website..

Maybe you remember a film about it: it was also called "Grounhog Day" (1993) with Bill Murray and Andie MacDowell. Here you are Roger Ebert's review.
I leave you the trailer:

I also leave a link to this year's forecast: Phil's forecast for 2011.

Information taken from Wikipedia.