Ever wonder how trick r' treating started? When you think about it, it's kind of a funny tradition, children (and sometimes adults) dressing up as all sorts of things and going door to door, begging for candy.
Once again we have to visit the Celts for our answer. During their festival of Samhain, the Celts believe that the barrier between the living and the dead was the thinnest, thus allowing all sorts of spirits passage to the living world. It was believed people who had passed on during that year, traveled to the "other world" on Halloween night. In researching, I've read that Celts would set a place at their tables in honor of passed loved ones, leaving offering of food. I also read that some would bury apples by the road side, to help out lost and wandering spirits.
There was also a belief in mischievous or vengeful spirits and prankster nature creatures such as trolls, that would torment the living. To deter these acts, people would dress as spirits, if they were out alone at dark, in order to fool the real spirits into believing that they were also a spirit, so that they would pass them by. In addition, offerings of fruit and nuts were left on door steps to please these malevolent spirits, so that they wouldn't play tricks on people of the house.
As Romans conquered the Celts and their traditions meshed, Catholicism influenced these traditions further. With the declaration of All Saints day, by Pope Boniface IV and All Souls day, by Pope Gregory III, the season became more of a celebration of prayer for the deceased. Souling became a tradition in 19th century Europe, where beggars would go door to door offering prayers for the deceased loved ones of the home-owner, in return for a soul cake. Soul cakes were sweet breads, seasoned with cinnamon and frequently had raisins or currents baked into the top of them, in the symbol of a cross. It was believed that souls spent a certain amount of time in purgatory before they passed on to heaven. In order to expedite the soul's passage to heaven, people prayed for them. The more prayers that the soul received, the quicker their assent to heaven.
Another tradition tied into Trick r' Treating's origins is guising. This was a tradition held in Scotland and Ireland, where children dressed as mischievous spirits (going along with the earlier Celt's beliefs) and went door to door, receiving fruit and nuts in return for their reciting a poem, song or joke. This act was representative of the the offerings that were left for the mischievous spirits, by the Celts, in hopes that they would pass their houses by and not play tricks on them.
Today, our tradition very closely represents guising, with the exception that we don't get a song or poem or joke told to us when we give the little spirits our offerings of sugary candy. Knowing that they go home on a major sugar high, maybe the parents should beg for the fruit and nut offerings of the past, to keep their hyper little spirits at bay ;)
Who knows how the tradition can develop from here. All that we Halloween lovers can hope for, is that it continues with some semblance of what it is now, or what it was originally.
Happy souling, guising, spirit deterring to you all :)