The groundhog Punxsutawney Phil in 2012

According to folklore, if it is cloudy when a groundhog emerges from its
burrow on this day, then spring will come early; if it is sunny, it will then
see its shadow and retreat back into its burrow, and the winter weather
will continue for six more weeks.

An early reference to Groundhog Day in America can be found in a diary
entry, dated February 4, 1841, of James Morris, a storekeeper in Berks
County, Pennsylvania.

The largest Groundhog Day celebration is held in Punxsutawney, Pa
where crowds as large as 40,000 have gathered to celebrate the
holiday since at least 1886.