According to Wikipedia, “Imbolc” comes from an Old Irish word meaning “in the belly.” Among agrarian people, Imbolc was associated with the onset of the lambing season.
The German settlers of Pennsylvania put candles in their windows and believed that if the weather was fair on Candlemas Day then the second half of winter would be stormy and cold. While this has always seemed counter-intuitive to me, this site explains the science of Groundhog Day and that cloudy weather is actually milder than clear and cold. It makes sense, then, that the shadow would portend six more weeks of winter. (A lifelong mystery is solved.)
The English and Scottish had wonderful sayings to mark this occasion:
The serpent will come from the hole
On the brown Day of Bride,
Though there should be three feet of snow
On the flat surface of the ground.
– Scottish saying
(Note the serpent instead of the groundhog.)
If Candlemas be fair and bright,
Winter has another flight.
If Candlemas brings clouds and rain,
Winter will not come again.
– English saying
Punxsutawney’s first Groundhog Day celebration was in 1886, and though other towns, particularly in the eastern US, have Groundhog Day ceremonies — Staten Island Chuck, anyone? — none are as famous as Punxsutawney’s. Some of this may lie with the groundhog’s official name, “Punxsutawney Phil, Seer of Seers, Sage of Sages, Prognosticator of Prognosticators, and Weather Prophet Extraordinary”. Still more popularity, and tourists, has come as a result of the movie Groundhog Day. The first official Groundhog Day prediction in Punxsutawney? No shadow – early Spring.