As with most major holidays, it's a little tricky to figure out exactly how Halloween came to be what it is today. From what we've gathered, All Hallows' Eve is a mix of religious holidays, harvest schedules, community building, and consumerism.
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Most people trace the ancient origins of Halloween back to the Celtic festival of Samhain. Two-thousand years ago, Celts celebrated their new year on November 1. On the night before the new year, they believed the boundary between the worlds of the living and the dead thinned. In the pre-industrial world, where wild things roamed in nights darker than you've ever seen (and people lacked corrective lenses), it's easy to understand how spooky superstitions could hold serious sway on a holiday such as these. Some Celts even wore costumes when they journeyed outside the house to avoid being recognized by ghosts.
In the 8th century CE, Pope Gregory III moved the Catholic Feast of All Martyrs - AKA All Saints Day or All-hallowmas - to November 1, making October 31st All Hallows' Eve. This tactic of appropriating native holidays was a common and clever move to spread Christianity. In the many centuries since its origins, the various other traditions of modern Halloween fell into place. Trick-or-treating may have developed from the All Souls' Day parade in England, which featured begging children who were rewarded with soul cakes. Jack O'Lanterns may have originated in Ireland, where they were a decoration to ward off evil that were inspired by the ghost story of Stingy Jack, a man who refused to pay the devil for a drink and, when he died, was forced to wander the countryside, banned from both heaven and hell.
Read more about the history of Halloween:
Mental Floss:What's the Origin of Jack-O'-Lanterns?
LiveScience: History of Halloween
Library of Congress: The Fantasy and Folklore of All Hallows