"black letter day" meaning in English

{
  "forms": [
    {
      "form": "black letter days",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "head": "black letter day"
      },
      "expansion": "black letter day (plural black letter days)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "Coordinate term: red letter day"
        },
        {
          "ref": "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",
          "text": "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.",
          "type": "quotation"
        },
        {
          "ref": "1893, Thomas De Quincey, “Conversation and Coleridge”, in Alexander H. Japp, editor, The Posthumous Works of Thomas De Quincey, volume II, London: William Heinemann",
          "text": "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.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "A day marked in ancient calendars in black letter, as distinguished from the more import red letter days; an inauspicious day."
      ],
      "id": "en-black_letter_day-en-noun-feQ6TAo6",
      "links": [
        [
          "black letter",
          "black letter"
        ],
        [
          "red letter day",
          "red letter day"
        ],
        [
          "inauspicious",
          "inauspicious"
        ]
      ],
      "synonyms": [
        {
          "word": "black-letter day"
        }
      ]
    }
  ],
  "word": "black letter day"
}

{
  "forms": [
    {
      "form": "black letter days",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "head": "black letter day"
      },
      "expansion": "black letter day (plural black letter days)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations"
      ],
      "examples": [
        {
          "text": "Coordinate term: red letter day"
        },
        {
          "ref": "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",
          "text": "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.",
          "type": "quotation"
        },
        {
          "ref": "1893, Thomas De Quincey, “Conversation and Coleridge”, in Alexander H. Japp, editor, The Posthumous Works of Thomas De Quincey, volume II, London: William Heinemann",
          "text": "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.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "A day marked in ancient calendars in black letter, as distinguished from the more import red letter days; an inauspicious day."
      ],
      "links": [
        [
          "black letter",
          "black letter"
        ],
        [
          "red letter day",
          "red letter day"
        ],
        [
          "inauspicious",
          "inauspicious"
        ]
      ]
    }
  ],
  "synonyms": [
    {
      "word": "black-letter day"
    }
  ],
  "word": "black letter day"
}