All Hallows' Eve, "Halloween", is the vigil of All Saints (All Hallows), the night of 31 October before the feast of 1 November, and forms the opening of the Christian triduum of the dead: Halloween, All Saints, and All Souls, three days on which the church remembers the holy dead and prays for all the departed. In Christian terms the eve's customs of light and remembrance belong to that commemoration, and in parts of the Catholic world the night is kept with prayer at graves.
The popular claim that Halloween simply is the Celtic festival of Samhain rechristened is more confidently asserted than the evidence allows. Samhain, the Gaelic feast at the start of winter, was indeed a time associated with the boundary between the living and the dead, and some Halloween folklore plausibly draws on it; but the fixing of All Saints to 1 November has its own documented Roman and Frankish history, and much of what is now taken as ancient, from trick-or-treat to the carved jack-o'-lantern, is modern and largely North American, developed among Irish and Scottish immigrant communities in the nineteenth and twentieth centuries. The honest account is a layered one: a Christian vigil, a possible Celtic substratum, and a heavily commercialised modern holiday, not a single pagan survival.