The official Pokémon website has announced that from December 17th, 2021 until December 26th, 2021, at 3:59 p.m. PST, players of Pokémon Sword and Pokémon Shield will find Starmie, Delibird, and Cryogonal more frequently in Pokémon Dens and they might even encounter a Shiny Delibird!

Starmie, Delibird, and Cryogonal Spread Cheer in Holiday Max Raid Battles

Celebrate the season in Pokémon Sword and Pokémon Shield, and you just might encounter a Shiny Delibird.

The holidays are here, and what better way to celebrate than with some Max Raid Battles? Until December 26, 2021, at 3:59 p.m. PST, Pokémon Sword and Pokémon Shield players will find Starmie, Delibird, and Cryogonal more frequently in Pokémon Dens. Very rarely during the event, you might even encounter a Shiny Delibird!

For an opportunity to encounter these Pokémon in a Max Raid Battle, you'll need to connect your Nintendo Switch system to the internet through Y-Comm or by selecting Mystery Gift in the X menu, then selecting Get the Wild Area News. After that, whenever you see a pillar of light shining from a Pokémon Den, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle with a Dynamax Pokémon. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/starmie-delibird-and-cryogonal-spread-cheer-in-holiday-max-raid-battles/