E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
All Hallows Day (November 1st).
The French call it Toussaint, which we have translated All Saints Day. Hallowmas is All-Saints festival. (Anglo-Saxon, hálig, but Hálig-mónáth was September, and Hálig-dg was simply a Holy-day.)