Halloween is All Hallows' Even, the eve of All Hallows, the old name for All Saints' Day on November 1. Hallow descends from Old English halga, a holy one, a saint. The Scots contraction Hallowe'en is attested in print by the 18th century, Robert Burns's 1785 poem being the famous early witness. The feast beneath it was fixed on November 1 by Rome in 835, after centuries of being celebrated in May.
In early Irish texts, Samhain, summer's end, was the hinge of the year: cattle driven down from pasture, fires lit on the hills, and the sidhe, the people of the mounds, abroad in the dark. The dead could cross, so the living wore disguises to be mistaken for them and left food out to buy peace. The story told ever since: the Church planted All Saints on that date to tame the older fire festival, and the masks and offerings simply changed their explanations.
The occult claim, from Victorian folklore revival to modern witchcraft: Samhain never died, it went underground, and October 31 remains the one night the boundary runs thin, when divination works, the dead answer, and the old powers still take tribute. On this reading the church feast is a lid on something still warm, and every costume is a survival, not a party choice.
A holy eve and a night of the dead share one date. Who moved in on whom, and does the answer change what you are celebrating?