CobbleScheduler is a Fabric server-side mod that lets server owners add calendar-based spawns for Cobblemon entities and their forms all managed through a single config.

If you want froms only spawns
Add to end of name ["name" = "pichu newyear"]
newyear being the custom form

how allowedBiomes should look
"allowedBiomes": [
"minecraft:forest",
"minecraft:plains"
]

Default Config:

{
  "holidays": [
    {
      "holiday": "New Year's Day",
      "startDate": "01-01",
      "pokemonEntityList": [
        {
          "name": "pichu",
          "level": 10,
          "spawn_rate": 0.6,
          "allowedBiomes": []
        },
        {
          "name": "clefairy",
          "level": 15,
          "spawn_rate": 0.4,
          "allowedBiomes": []
        }
      ],
      "holidayMessage": "Happy New Year! Celebrate with a special Pichu!"
    },
    {
      "holiday": "March Break",
      "startDate": "03-10",
      "endDate": "03-14",
      "pokemonEntityList": [
        {
          "name": "togekiss",
          "level": 15,
          "spawn_rate": 0.7,
          "allowedBiomes": []
        },
        {
          "name": "leafeon",
          "level": 40,
          "spawn_rate": 0.3,
          "allowedBiomes": []
        }
      ],
      "holidayMessage": "Enjoy your March Break Pokemon Spawns!"
    },
    {
      "holiday": "Independence Day",
      "startDate": "07-04",
      "pokemonEntityList": [
        {
          "name": "pikachu",
          "level": 25,
          "spawn_rate": 0.7,
          "allowedBiomes": []
        },
        {
          "name": "charizard",
          "level": 40,
          "spawn_rate": 0.3,
          "allowedBiomes": []
        }
      ],
      "holidayMessage": "Celebrate freedom with a Pikachu and Charizard!"
    },
    {
      "holiday": "Thanksgiving",
      "startDate": "11-25",
      "pokemonEntityList": [
        {
          "name": "farfetchd",
          "level": 20,
          "spawn_rate": 0.5,
          "allowedBiomes": []
        },
        {
          "name": "squirtle",
          "level": 10,
          "spawn_rate": 0.5,
          "allowedBiomes": []
        }
      ],
      "holidayMessage": "Give thanks with Farfetch'd and Squirtle!"
    },
    {
      "holiday": "Halloween",
      "startDate": "10-31",
      "pokemonEntityList": [
        {
          "name": "gastly",
          "level": 25,
          "spawn_rate": 0.6,
          "allowedBiomes": []
        },
        {
          "name": "pumpkaboo",
          "level": 20,
          "spawn_rate": 0.4,
          "allowedBiomes": []
        }
      ],
      "holidayMessage": "Spooky season with Gastly and Pumpkaboo!"
    },
    {
      "holiday": "Christmas",
      "startDate": "12-25",
      "pokemonEntityList": [
        {
          "name": "delibird",
          "level": 15,
          "spawn_rate": 0.7,
          "allowedBiomes": [
            "minecraft:forest",
            "minecraft:plains"
          ]
        },
        {
          "name": "snorlax",
          "level": 35,
          "spawn_rate": 0.3,
          "allowedBiomes": [
            "minecraft:forest",
            "minecraft:plains"
          ]
        }
      ],
      "holidayMessage": "Merry Christmas with Delibird and Snorlax!"
    }
  ],
  "cooldown": 7200000,
  "sendMessagesEnabled": false,
  "messageCooldown": 7200000
}