"first-day cover" meaning in English

{
  "forms": [
    {
      "form": "first-day covers",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "first-day cover (plural first-day covers)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "first day cover"
        }
      ],
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Philately",
          "orig": "en:Philately",
          "parents": [
            "Hobbies",
            "Recreation",
            "Human activity",
            "Human behaviour",
            "Human",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Alternative form of first day cover."
      ],
      "id": "en-first-day_cover-en-noun-zR1DBTYV",
      "links": [
        [
          "philately",
          "philately"
        ],
        [
          "first day cover",
          "first day cover#English"
        ]
      ],
      "raw_glosses": [
        "(philately) Alternative form of first day cover."
      ],
      "tags": [
        "alt-of",
        "alternative"
      ],
      "topics": [
        "hobbies",
        "lifestyle",
        "philately"
      ]
    }
  ],
  "word": "first-day cover"
}

{
  "forms": [
    {
      "form": "first-day covers",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "first-day cover (plural first-day covers)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "first day cover"
        }
      ],
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Philately"
      ],
      "glosses": [
        "Alternative form of first day cover."
      ],
      "links": [
        [
          "philately",
          "philately"
        ],
        [
          "first day cover",
          "first day cover#English"
        ]
      ],
      "raw_glosses": [
        "(philately) Alternative form of first day cover."
      ],
      "tags": [
        "alt-of",
        "alternative"
      ],
      "topics": [
        "hobbies",
        "lifestyle",
        "philately"
      ]
    }
  ],
  "word": "first-day cover"
}