All Saints Day, observed on November 1 and established in the eighth century by Pope Gregory III, for the purpose of honoring all Saints.
The day came to incorporate some of the ancient traditions of the Celtic festival of Samhein (pronounced sow-in). The Celtic New year was celebrated on November 1 and the Samhein festival marked the end of the summer and harvest and the beginning of a dark, cold winter. The Celts would light bonfires and wear costumes to ward off ghosts.
The evening before All Saints Day was known as All Hallows Eve and later Halloween.
For more information:
www.history.com/topics/Halloween/history-of-halloween#section_3