I can’t seem to find a source that gives me one origin for the whole “trick-or-treating” thing, but I found a bunch of different traditions that I think are awesome. All over Western Europe people played tricks, thought the spirits played tricks, and went around asking for gifts (sometimes in costume and sometimes not).  Celts offered treats to avoid spirits playing tricks and Druids asked for favours in return for protection of the soul.  By far, my favourite ancient custom is the Irish or English version of “soul-caking”.  You can get the recipe here: http://www.catholicculture.org/culture/liturgicalyear/recipes/view.cfm?id=1378. Essentially, “soulers” went around and sang their songs without stopping or having much tonal variation…