Halloween originated from the ancient Celtic festival of Samhain, where the Celts believed the boundary between the living and the dead became blurred and spirits could cause harm. They would light bonfires and wear costumes to ward off spirits. The tradition was incorporated into the Roman harvest festival of Pomona. In the 8th century, Pope Gregory III designated November 1 as a time to honor all saints and martyrs, All Hallows' Day incorporated pagan traditions. When Irish immigrants fled the potato famine, they brought Halloween customs to America in the 19th century, where it is now celebrated as a time for costumes, parties, trick-or-treating and scary decorations.