Groundhog Day is a tradition observed regionally in the United
States and Canada on February 2 of every year. It derives from
the Pennsylvania Dutch superstition that if a groundhog emerges
from its burrow on this day and sees its shadow, it will retreat to
its den and winter will go on for six more weeks; if it does not
see its shadow, spring will arrive early.

The observance of Groundhog Day in the United States first
occurred in German communities in Pennsylvania on
February 2, 1840.
.

While the tradition remains popular in the 21st century,
studies have found no consistent association between
a groundhog seeing its shadow and the subsequent
arrival time of spring-like weather.

(From Wikipedia, the free encyclopedia)