It all began in what is now France and the British Isles. It was started by the Celtic people long ago....
The Celtic people [clue] the night of October 31. It was the night of their festival of Samhain, Lord of the Dead. The Celtics feared Samhain and in order to please him, the Druids (Celtic priests) held cruel fire rites in his honor. They made [clue] bonfires every year at this time. The bonfires were usually made of oak and considered sacred.
At this festival, the Druids made sacrifices to the gods. They burned criminals, prisoners, and animals alive. By observing the victims die, the Druids saw omens of the future, both good and bad. The powers to make predictions of the future were thought to be strongest on this night. Why? It was because the Druids believed that the spirits were allowed to roam free on this night of the year. The Druids believed the spirits were [clue] and wild on this night, and if they were treated right, they would help with predictions of the future. However, if the spirits were ignored, they would punish the Celtic community.
The Druids believed that, above powerful and wild, some spirits were [clue]. This is why the Celtic people wore costumes during the Samhain festival. The purpose of the costumes was to frighten the spirits. Plus, this way, the spirits wouldn't be able to recognize them.
(This is the origin of costumes.)
To please the spirits, the Celtic people left food outside their houses on Halloween. If any [clue] spirits came by, they could take the food and leave the Celtics in peace.
(This is the origin of 'trick or treat'ing.)