Halloween originated as the ancient Celtic festival of Samhain, where the Celts believed the ghosts of the dead returned to earth on October 31st. People wore costumes to avoid being recognized by ghosts. As Christianity spread, the holiday was changed to All Saints' Day on November 1st. Over time, traditions evolved including trick-or-treating, where people would receive food and money in exchange for prayers. Irish immigrants brought Halloween traditions to America in the 19th century, where it became a night of wearing costumes and collecting candy.