Trick-or-treating is a customary celebration for children on Halloween. Children go in costume from house to house, asking for treats such as candy or sometimes money, with the question, "Trick or treat?" The word "trick" implies a "threat" to perform mischief on the homeowners or their property if no treat is given. The practice is said to have roots in the medieval practice of mumming, which is closely related to souling. John Pymm writes that "many of the feast days associated with the presentation of mumming plays were celebrated by the Christian Church." These feast days included All Hallows' Eve, Christmas, Twelfth Night and Shrove Tuesday. Mumming practiced in Germany, Scandinavia and other parts of Europe, involved masked persons in fancy dress who "paraded the streets and entered houses to dance or play dice in silence".
The practice may have originated in a Celtic festival, held on 31 October–1 November, to mark the beginning of winter. It was called Samhain in Ireland, Scotland and the Isle of Man, and Calan Gaeaf in Wales, Cornwall and Brittany. The festival is believed to have pre-Christian roots. After the Christianization of Ireland in the 5th century, some of these customs may have been retained in the Christian observance of All Hallows' Eve in that region—which continued to be called Samhain/Calan Gaeaf—blending the traditions of their ancestors with Christian ones. It was seen as a liminal time, when the spirits or fairies (the Aos Sí), and the souls of the dead, could more easily come into our world. It was believed that the Aos Sí needed to be propitiated to ensure that the people and their livestock survived the winter.