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Halloween has its origins in the Catholic Church. The word comes from a contracted corruption of All Hallows Eve. November 1st, "All Hollows Day" also called "All Saints Day," is a Catholic day of observance in honor of saints.
In the 5th century BC, in Celtic Ireland, summer officially ended on October 31. The holiday was called Samhain (sow-en), the Celtic New year. The disembodied spirits of all those who had died throughout the year would return in search of living bodies to possess for the next year. It was believed to be their only hope for the afterlife. The Celts also believed that all laws of space and time were suspended during this time which allowing the spirit world to intermingle with the living.
The “Trick or treat” tradition is innocent fun on the Halloween Day. It comes from the Catholic tradition of eating and giving soul cakes (small round cakes) on Halloween. The Jack-o-lantern comes from Irish folklore. Wearing costumes comes from the days when Celtic villagers would dress up in ghoulish costumes and paraded around noisily, being as destructive as possible in order to frighten away spirits looking for bodies to possess.