variant spelling of even (adj.), now archaic or poetic. E'enamost "even almost" is recorded from 1735 in Kentish speech.
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, or Holy 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. Otherwise obsolete hallow (n.) "holy person, saint," is from the source of hallow (v.). Also see even (n.), and compare hallows. Hallow-day for "All-Saints Day" is from 1590s; earlier was halwemesse day (late 13c.).
in good den, found in the early dramatists, a contraction of good e'en "good evening;" the phrase was short for God give you good den.
Who would linger by the fire, nor from toil an hour snatch
When villages play football in a merry monster match;
E'en a mere ale-drinking Saxon feels some fervour in his soul
As he watches and bets glasses on a drop-kick at the goal.
[from "A Lay of English Field Sports," by "Colonel Chasse," in The Sporting Review, June 1849]