The Celtic festival from which Halloween costume traditions originated is Samhain.
Samhain was a Gaelic festival traditionally observed around October 31 and November 1, marking the transition into winter. In Irish, Scottish, and Manx tradition, it was viewed as a liminal time when supernatural beings and the dead could more readily enter the human world. Disguises and costumes were associated with protection, imitation, or concealment from these beings.
By at least the early modern period, mumming and guising involved going door to door in costume, reciting verses or songs in exchange for food. These customs helped shape later Halloween practices, especially after Irish and Scottish immigrants carried them to North America.
Samhain is sometimes loosely described as an ancient “Celtic New Year,” but that interpretation is disputed by historians. Halloween also developed through interaction with Christian observances such as All Hallows’ Eve, so Samhain is the key ancestral festival, not the sole source of every modern Halloween custom.