The document summarizes the origin and history of Halloween. It began as an ancient Celtic festival held on October 31st, which marked the boundary between the living world and the spirit world. When the Romans conquered the Celts in the 1st century AD, their festivals were combined. In the 800s, the Catholic Church established All Saints Day on November 1st and All Souls Day on November 2nd, which eventually merged to become Halloween. Halloween was brought to America by European settlers and became a secular holiday celebrated with costumes, pumpkins, and trick-or-treating in the 19th-20th centuries.