Groundhog Day can be traced to an ancient German tradition of using badgers to predict the weather. It was adopted by Christians as part of the festivities concerning Candlemas, which is today, Feb. 2.
The Germans who migrated to Pennsylvania apparently didn’t feel like dealing with badgers so they substituted a groundhog and hence the American tradition.
The earliest reference to a groundhog making a prediction regarding the length of winter might be a Feb. 4, 1841 dairy entry by Berks County storekeeper James Morris who noted it as part of the Germans Candlemas traditions from two days earlier.
Regarding a clear day meaning a longer winter, there is an old English poem:
If Candlemas be fair and bright,
Winter has another flight,
If Candlemas brings cold and rain,
Winter will not come again.