Halloween originated from the ancient Celtic festival of Samhain, where people would light bonfires and wear costumes to ward off ghosts. Pope Gregory III designated November 1st as All Saints' Day and incorporated traditions such as dressing as ghosts. Over time, Halloween evolved into a holiday with activities like trick-or-treating. Ancient Gaels believed that on October 31st, the worlds of the living and the dead overlapped and spirits would cause havoc. Masks and costumes were worn to mimic evil spirits so they would think people were one of them.