Most experts believe that the tradition of wearing costumes on Halloween originated in ancient days with the Celtic festival of Samhain, where participants lit fires and wore costumes to scare away ghosts.
Approximately 2,000 years ago, the Celts lived in an area now occupied by Ireland, northern France and the United Kingdom. The Celtic new year was Nov. 1, which marked the end of the harvest season and the beginning of the dark days of winter, signifying death. To them, the night before the new year was a time when the veil between the living and the dead was the thinnest, allowing spirits to cross over. The Celts celebrated Samhain on Oct. 31 to pay homage to these spirits, which they both feared and respected.
Modern Halloween practices are a combination of Celtic, ancient Roman, Catholic and pagan traditions. After the Roman Empire conquered this area, they combined two Roman festivals with Samhain practices. The word "Halloween" has Catholic roots. In the eighth century, Pope Gregory III designated Nov. 1 as a time to honor all saints and martyrs and called it All Saints' Day. The night before Nov. 1 was called All Hallows' Eve, which eventually became known as Halloween.