Where does the custom of “trick or treating” come from? Is it really ancient, a few centuries old, or relatively modern? Let’s look at the evidence:

Kevin Danaher, in his remarkable book The Year in Ireland, has a long discussion of the traditional Irish celebrations of this festival. In one section on “Hallow-E’en Guisers,” he says:

A familiar sight in Dublin city on and about October 31 is that of small groups of children, arrayed in grotesque garments and with faces masked or painted, accosting the passers-by or knocking on house doors with the request: “Help the Hallow E’en party! Any apples or nuts?” in the expectation of being given small presents; this, incidentally, is all the more remarkable as it is the only folk custom of the kind which has survived in the metropolis.

Borrowed from http://www.neopagan.net/Halloween-Origins.html