Brewer's: Lammas Day
(August 1st) means the loaf-mass day. The day of first-fruit offerings, when a loaf was given to the priests in lieu of the first-fruits. (Saxon, hlam-mæsse, for hlaf-mæsse dag.)
August I Old Style, August 12 New Style.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894