Costumes
The belief was that on this day, our world and the spirit world were at their closest, possibly allowing the spirits of the dead to cross over. Those leading the rites would wear masks to disguise themselves as spirits in the hopes of being able to mingle with them without being carried off to the Netherworld. Eventually as the religious aspects fell away from the celebrations, everyday people took up the wearing of costumes, depicting all sorts of folkloric characters.
Costumes for Halloween as well as trick-or-treating took off in America after the great migrations to our country in the mid-late 1800’s. Most of the old Druid/Celtic traditions came over with the Irish and the Scots.