Saint Martin's Day or Martinmas (obsolete: Martlemas), and historically called Old Halloween or Old All Hallows Eve, is the feast day of Saint Martin of Tours and is celebrated in the liturgical year on 11 November. In the Middle Ages and early modern period, it was an important festival in many parts of Europe, particularly Germanic-speaking regions. In these regions, it marked the end of the harvest season and beginning of winter and the "winter revelling season". Traditions include feasting on 'Martinmas goose' or 'Martinmas beef', drinking the first wine of the season, and mumming. In some German and Dutch-speaking towns, there are processions of children with lanterns (Laternelaufen), sometimes led by a horseman representing St Martin. The saint was also said to bestow gifts on children. In the Rhineland, it is also marked by lighting bonfires.

Martin of Tours (died 397) was a Roman soldier who was baptized as an adult and became a bishop in Gaul. He is best known for the legend whereby he cut his cloak in half with his sword to give half to a beggar who was dressed in only rags in the depth of winter. That night Martin had a vision of Jesus Christ wearing the half-cloak.

In his study Medieval English Martinmesse: The Archaeology of a Forgotten Festival, Martin Walsh suggests that 'Martinmas' had roots in a pre-Christian festival marking the end of the harvest season and beginning of winter. In the ancient Roman world, 10 November was reckoned as the beginning of winter, while among the Insular Celtic peoples, it was marked by the 1 November festival of Samhain/Calan Gaeaf.

Traditionally, in many parts of Europe, St Martin's Day marked the end of the harvest and the beginning of winter. The feast coincides with the end of the Octave of Allhallowtide.

Martinmas was traditionally when livestock were slaughtered for winter provision. It may originally have been a time of animal sacrifice, as the Old English name for November was Blōtmōnaþ ('sacrifice month').

Goose is eaten at Martinmas in most places. There is a legend that St Martin, when trying to avoid being ordained bishop, hid in a pen of geese whose cackling gave him away. Once a key medieval autumn feast, a custom of eating goose on the day spread to Sweden from France. It was primarily observed by the craftsmen and noblemen of the towns. In the peasant community, not everyone could afford this, so many ate duck or hen instead.

In winegrowing regions of Europe, the first wine was ready around the time of Martinmas. Although there was no mention of a link between St Martin and winegrowing by Gregory of Tours or other early hagiographers, St Martin is widely credited in France with helping to spread winemaking throughout the region of Tours (Touraine) and facilitating vine-planting. The old Greek tale that Aristaeus discovered the advantage of pruning vines after watching a goat, has been appropriated to St Martin. He is credited with introducing the Chenin blanc grape, from which most of the white wine of western Touraine and Anjou is made.

Bonfires are lit on St Martin's Eve in the Rhineland region of Germany. In the fifteenth century, these bonfires were so numerous that the festival was nicknamed Funkentag (spark day). In the nineteenth century it was recorded that young people danced around the fire and leapt through the flames, and that the ashes were strewn on the fields to make them fertile.