Groundhog Day (Pennsylvania German: Grund'sau dåk, Grundsaudaag, Grundsow Dawg, Murmeltiertag; Canadian French: Journée de la marmotte; Lunenburg, Nova Scotia: Daks 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. In 2026, the shadow was seen by Punxsutawney Phil, auguring six more weeks of winter.

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. The weather lore was brought from German-speaking areas where the badger (German: Dachs) is the forecasting animal, while in Hungary for example the bear serves the same purpose, and badgers were only watched when bears were not around. It is related to the lore that clear weather on the Christian festival of Candlemas predicts a prolonged winter.

The Groundhog Day ceremony held at Punxsutawney in western Pennsylvania, centering on a groundhog named Punxsutawney Phil, has become the most frequently attended ceremony. Grundsow Lodges in Pennsylvania Dutch Country in the southeastern part of the state observe the occasion as well. Other cities in the United States and Canada also have adopted the event.

The Pennsylvania Dutch were immigrants from German-speaking areas of Europe. A German tradition marked Candlemas (February 2) as "Badger Day" (Dachstag): if a badger emerging from its sett encountered sunshine, and thereby cast a shadow, four more weeks of winter were predicted.

Various traditional superstitions in German folk religion continue to be linked with Candlemas, although this was discouraged by the Reformed Churches in the 16th century. Notably, several traditions that are part of weather lore use the weather at Candlemas to predict the start of spring. The weather-predicting animal on Candlemas was usually the badger, although regionally, the animal was the bear or the fox.

The original weather-predicting animal in Germany had been the bear, another hibernating mammal, but when they grew scarce, the lore became altered. Similarity to the groundhog lore has been noted for the German formula: Sonnt sich der Dachs in der Lichtmeßwoche, so geht er auf vier Wochen wieder zu Loche ("If the badger sunbathes during Candlemas-week, for four more weeks he will be back in his hole"). A slight variant is found in a collection of weather lore (Bauernregeln, lit. "farmers' rules") printed in Austria in 1823.

The Pennsylvanians maintained the same tradition as the Germans on Groundhog Day, except that winter's spell would be prolonged for six weeks instead of four. For the Pennsylvania Dutch, the badger became the dox, which in Deitsch referred to "groundhog".

The standard term for "groundhog" was grun'daks (from German dachs), with the regional variant in York County being grundsau, a direct translation of the English name, according to a 19th-century book on the dialect. The form was a regional variant according to one 19th-century source. However, the weather superstition that begins Der zwet Hær'ning is Grund'sau dåk. Wânn di grundsau îr schâtte sent... ("February second is Groundhog day. If the groundhog sees its shadow...") is given as common to all fourteen counties in Dutch Pennsylvania Country, in a 1915 monograph.