Haloween is more about the fall harvest, at least in my interpretation. The costumes are just the fun. Some do get gross though.
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.