From the practice of marking in red the holy days in church calendars
American Heritage Dictionary of the English Language, 5th Edition
Red-letter Sentence Examples
endowment misselling claims to be submitted within three years of receipt of the first red letter.
red-letter day for all the wrong reasons, leaving you wishing you could turn the clock back.
Those are red-letter days in our lives when we meet people who thrill us like a fine poem, people whose handshake is brimful of unspoken sympathy, and whose sweet, rich natures impart to our eager, impatient spirits a wonderful restfulness which, in its essence, is divine.
Indeed, Friday could prove a red-letter day for all the wrong reasons, leaving you wishing you could turn the clock back.