Though Groundhog Day proper is a North American tradition of fairly recent origin, its historical roots stretch back through the medieval Christian feast of Candlemas to ancient pagan midwinter celebrations. For the past 100 years, Groundhog Day has revolved around the ceremonial de-hibernation of a domesticated Pennsylvania woodchuck named Punxsutawney Phil.
Every February, Phil is hauled out of his artificial burrow on Gobbler's Knob (a hilltop near Punxsutawney, PA) to "predict" the next six weeks' weather. If Punxsutawney Phil sees his shadow, legend has it, the cold winter weather will continue into March; if he doesn't, there'll be an early spring.
True? Of course not. It's folklore. Historically, the groundhog's "predictions" have more often been wrong than right.