There was an article on the History Channel website about Halloween. Basically it stated that Halloween fell about the time of the Celtic New Year. They believed that as the old year passed away and the new year opened it was a time when the spirit and mortal world were open. They would dress up so as to confuse the spirits.
I was told a story with no documentation that it was based on the idea of the harvest, and the chidren of poor families went to the farmers houses dressed to conceal their identities to receive part of the different farmers harvest. Early time welfare??? Who knows.