Halloween had its beginnings in an ancient, pre-Christian Celtic festival of the dead. The Celtic peoples, who were once found all over Europe, divided the year by four major holidays. According to their calendar, the year began on a day corresponding to November 1st on our present calendar. The date marked the beginning of winter. Since they were pastoral people, it was a time when cattle and sheep had to be moved to closer pastures and all livestock had to be secured for the winter months. Crops were harvested and stored. The date marked both an ending and a beginning in an eternal cycle.
The Celts believed that there is a material world that we live in, and a supernatural world occupied by strange creatures, gods, elves, and the spirits of the dead. Twice a year (end of February and end of October), the borders between these worlds become thin, and you can pass between the worlds.
The Celts thought that, on the night of October 31, supernatural creatures would pass through the veil between worlds and would walk our world.
Some people feared that ghosts and magical creatures might cause trouble. So the people dressed up in costumes. Some believed that the costumes would frighten away the creatures. Others believed that the magical creatures would be fooled into thinking that the costumed people were also magical – and leave them alone.
The Celts also left out donations of food and fruit on their doorsteps. Some hoped that this would attract good spirits. Others thought that it would satisfy bad spirits and make them go away.
Over time, the two traditions of costumes and food turned into the tradition of children dressing up in costume to ask for candy. Pretending to be magical creatures, they threaten to play a trick on you, unless you give them an offering - "Trick or Treat!"