Halloween (n.) Look up Halloween at Dictionary.com
also Hallow-e'en, Hallow e'en, 1781, in a Scottish context, the word and the magical lore about the date were popularized by Burns' poem (1785, and he attached a footnote explaining it), but it probably dates to 17c. in Scotland and is attested as the name of a tune in 1724. The tune is mentioned again in an English-Scots songbook ("The Chearful Companion") in 1783, and Burns was not the first to describe the customs in print.
Hallow-E'en, ?r ?ol? Eve, is the evening previous to the celebration of All Saints. That it is propitious to the rites of divination, is an opinion still common in many parts of Scotland. [John Main, footnote to his poem "Hallow-E'en," Glasgow, 1783]
It is a 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 Day" is from 1590s; earlier was halwemesse day (late 13c.).