Groundhog Day is celebrated on February 2 each year in the United States and Canada. On that day tradition holds that the lowly groundhog will emerge from his winter hibernation in his den, look around, and determine whether or not it's time to wake up. If the sun is shining and he sees his shadow, winter will return for six more weeks. If the groundhog can't see his shadow, spring has arrived.

So....where did THIS idea come from? hmmm? It's actually based on a couple of traditions started in Europe. Candlemas day, a day of ritual purification for Mary, falls 40 days after the birth of Jesus: February 2nd. For centuries the custom was to have the clergy bless candles, which were then distributed to the people to be displayed in their windows to light up the dark of winter. Even then, it marked a milestone in the winter and the weather that day was important. According to an old English song:
If Candlemas be fair and bright,
Come, Winter, have another flight;
If Candlemas brings clouds and rain,
Go Winter, and come not again.
According to an old Scotch couplet:
If Candlemas Day is bright and clear,
There'll be twa (two) winters in the year.

The Pagan celebration of Imbolc, falling halfway between the Winter Solstice and the Spring Equinox also falls on February 2nd. The Pagans believed that if the weather was fair, the second half of Winter would be stormy and cold.

So, how did this tradition get to the United States and Canada? The Roman legions, during the conquest of the northern country, supposedly brought this tradition to the Teutons (Germans), who adopted it and concluded that if the sun made an appearance on Candlemas Day an emerging hedgehog,would cast a shadow, thus predicting six more weeks of bad weather. Germans were the earliest settlers in Pennsylvania and there was a short supply of hedgehogs there to fulfill the tradition. They did find groundhogs in profusion in many parts of the state, however. They determined that the groundhog, like the European hedgehog, was a most intelligent and sensible animal and therefore decided that if the sun did appear on February 2nd, so wise an animal as the groundhog would see its shadow and hurry back into its underground home for another six weeks of winter. The Germans recited:
For as the sun shines on Candlemas Day,
So far will the snow swirl until the May.

Groundhog Day grew popular in the United States during the late eighteen hundreds due to the efforts of Clymer H. Freas, a newspaper editor, and W. Smith, a American congressman and newspaper publisher. They organized and popularized a yearly festival in Punxsutawney, Pennsylvania where a groundhog named Punxsutawney Phil was used to foretell how much longer winter would last. This very popular event is still being held and is called Groundhog Day. In Canada the name of the groundhog that is used to predict the length of winter is Wiarton Willy.

Sources:
1. COMPTON'S INTERACTIVE ENCYCLOPEDIA, 1995
2. http://www.gojp.com/groundhog/
3. http://www.friesian.com/grndhog.htm
4. http://wilstar.com/holidays/grndhog.htm