Samhain, (Celtic: “End of Summer”) also spelled Samain, in ancient Celtic religion, one of the most important and sinister calendar festivals of the year. At Samhain, held on November 1, the world of the gods was believed to be made visible to humankind, and the gods played many tricks on their mortal worshippers; it was a time fraught with danger, charged with fear, and full of supernatural episodes. Sacrifices and propitiations of every kind were thought to be vital, for without them the Celts believed they could not prevail over the perils of the season or counteract the activities of the deities. Samhain was an important precursor Halloween.

From 2023, Samhain is a yearly public holiday in the Republic of Ireland, to mark the seasonal festival. A government statement noted that all four of the traditional Celtic seasonal festival will now be public holidays.