Final answer:
Halloween evolved from the ancient Celtic festival of Samhain and merged with the Christian celebration of All Saints' Day.
Step-by-step explanation:
Halloween gradually evolved from the ancient Celtic festival of Samhain. Samhain was a celebration that marked the end of the harvest season and the beginning of winter. As Christianity spread throughout Europe, the holiday merged with the Christian celebration of All Saints' Day on November 1st, which is also known as All Hallows' Day. The evening before All Hallows' Day became known as All Hallows' Eve, which eventually transformed into Halloween.
Learn more about The history and evolution of Halloween