The story
From all + plural of hallow (“saint”). Forms in -n (All-Hallown etc.) show reflex of the original genitive plural (Old English halgena).
First known use, by meaning
- from 10th c. The saints, taken collectively.
- from 10th c. All Saints' Day, the 1st of November; the Christian feast day honoring all Christian saints.
Etymology from Wiktionary, CC BY-SA 4.0.