black letter day

plural black letter days · alternative black-letter day

1. A day marked in ancient calendars in black letter, as distinguished from the more important red letter days; an inauspicious day.

   But the monsters who interrupt men in the middle of a sentence are to be found everywhere; and they are always practising. Red-letter days or black-letter days, festival or fast, makes no difference to them.

   1893, Thomas De Quincey, “Conversation and Coleridge”, in Alexander H. Japp, editor, The Posthumous Works of Thomas De Quincey, volume II, London: William Heinemann

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   So pray consider the kill-joy proposition as entirely retracted, and give us something of yourselves, only on simple black-letter days when the Herald Angels have not been raising expectations early in the morning.

   1877 December 3, George Eliot, “Letter to Mrs. Burne-Jones”, in J. W. Cross, editor, George Eliot's Life, as Related in Her Letters and Journals, volume III, published 1885, page 233