Halloween (n.) Look up Halloween at Dictionary.com
also Hallow-e'en, c. 1745, Scottish shortening of Allhallow-even "Eve of All Saints, last night of October" (1550s), the last night of the year in the old Celtic calendar, where it was Old Year's Night, a night for witches. A pagan holiday given a cursory baptism. See hallow (n.) + even (n.); also see hallows. Hallow-day for "All-Saints'" is from 1590s.