Black Letter Day
An unlucky day; one to be recalled with regret. The Romans marked their unlucky days with a piece of black charcoal, and their lucky ones with white chalk.
By fell black-letter dogs ... That from Gothic kennels eager strut.
Matthias: Pursuits of Literature.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Black Letter Day from Infoplease: