from the Invention of the Cross to St. Helen's Day (May 3rd to August 18th). Not Christ-mas, but Cross-mas. Rogation Sunday is called Crouchmas Sunday, and Rogation week is called Crouchmas.
From bull-cow fast, Till Crouchmas be past
[i.e. August 18th].
Tusser: May Remembrances.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Crouchmas from Infoplease: