"Sunday letter" meaning in English

{
  "forms": [
    {
      "form": "Sunday letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "Sunday letter (plural Sunday letters)",
      "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"
        }
      ],
      "glosses": [
        "A dominical letter."
      ],
      "id": "en-Sunday_letter-en-noun-vcny8Jwe",
      "links": [
        [
          "dominical letter",
          "dominical letter"
        ]
      ]
    }
  ],
  "word": "Sunday letter"
}

{
  "forms": [
    {
      "form": "Sunday letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "Sunday letter (plural Sunday letters)",
      "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"
      ],
      "glosses": [
        "A dominical letter."
      ],
      "links": [
        [
          "dominical letter",
          "dominical letter"
        ]
      ]
    }
  ],
  "word": "Sunday letter"
}