The origins of Halloween lie in an ancient Celtic festival known as Samhain, which was a festival of the dead. The festival was traditionally held on the day of the official start of winter on the Celtic calendar, which corresponds to November 1 on the modern calendar.
During Samhain, revelers would light bonfires to honor the dead and keep them from the realm of the living. It was believed that during Samhain, spirits were able to visit the world of the living. It was also the time when those who had passed away during the year could travel to the beyond.
Christian missionaries have a lot to do with Halloween as it is known in the modern world. In an attempt to diminish the celebration of Samhain, they assigned the same day as the feast of All Saints, or All Hallows Day, on November 1. "Hallow" means holy or sanctified. Over the years, the symbolism of Samhain remained, and the celebration of All Hallow's Eve continued as a celebration of the disembodied dead. Gifts of food and drink were placed out for these spirits to appease them. This began the tradition of children dressing up as spooks and ghouls and going door to door to collect treats.