Groundhog Day (Pennsylvania German: Grund’sau dåk, Grundsaudaag, Grundsow Dawg, Murmeltiertag; Nova Scotia: Daks Day) is a popular tradition celebrated in the United States on February 2.

It derives from Pennsylvania Dutch superstition that if a groundhog (Deitsch: grundsau, grun′daks, dox) emerging from its burrow on this day sees a shadow, it will retreat back into its den and winter weather will persist for six more weeks, and if not, spring season will arrive early.

Source: Wikipedia