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Halloween is an annual holiday celebrated each year on October 31, and Halloween 2019 occurs on Thursday, October 31. It originated with the ancient Celtic festival of Samhain, when people would light bonfires and wear costumes to ward off ghosts. In the eighth century, Pope Gregory III designated November 1 as a time to honor all saints; soon, All Saints Day incorporated some of the traditions of Samhain. The evening before was known as All Hallows Eve, and later Halloween. Over time, Halloween evolved into a day of activities like trick-or-treating, carving jack-o-lanterns, festive gatherings, donning costumes and eating sweet treats. - courtesy of History Channel - https://www.history.com/topics/halloween/history-of-halloween
The tradition of dressing in costume for Halloween has both European and Celtic roots. Hundreds of years ago, winter was an uncertain and frightening time. Food supplies often ran low and, for the many people afraid of the dark, the short days of winter were full of constant worry.
On Halloween, when it was believed that ghosts came back to the earthly world, people thought that they would encounter ghosts if they left their homes. To avoid being recognized by these ghosts, people would wear masks when they left their homes after dark so that the ghosts would mistake them for fellow spirits.
On Halloween, to keep ghosts away from their houses, people would place bowls of food outside their homes to appease the ghosts and prevent them from attempting to enter.