The first Groundhog Day was celebrated on February 2, 1887 by a "rodent meteorologist". This tradition continues to this day, where if a groundhog comes out of its hole from hibernation, and sees its shadow, it will get scared and run back into its hole. This signifies there will be 6 more weeks of Winter. If he does not see his shadow, there will be an early Spring.

This was originally a German tradition, where they used hedgehogs instead, but switched to groundhogs once they arrived to the United States, as they were much more plentiful.

More Info: www.history.com