A saint’s day or holiday, marked in the calendars with red letters. Red letter men; Roman Catholics: from their observation of the saint days marked in red letters.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Red Lattice * Red RagNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes