The wearing of costumes at Halloween may come from the belief that supernatural beings, or the souls of the dead, roamed the earth at this period.
The practice may have originated in a Celtic festival, held upon October 31 to November 1, to mark the arrival 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 sticking to of All Hallows' Eve in that region which continued to be called Samhain/Calan Gaeaf blending the traditions of their ancestors as soon as Christian ones.