The official Pokémon website has announced that during January 2021, Normal-type and Dragon-type Pokémon will be frequenting Pokémon Sword and Pokémon Shield’s Pokémon Dens and there is a greater chance that players will encounter Gigantamax Snorlax and Gigantamax Duraludon.

Gigantamax Snorlax and Gigantamax Duraludon in January Max Raid Battles

During January, Normal-type and Dragon-type Pokémon will be frequenting Pokémon Sword and Pokémon Shield’s Pokémon Dens.

You'd best not be draggin' your feet while playing Pokémon Sword or Pokémon Shield this January. All month long, you'll find Normal-type and Dragon-type Pokémon much more frequently in Max Raid Battles. And be on guard, because there's a greater chance that you'll encounter Gigantamax Snorlax and Gigantamax Duraludon.

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. Usually, there'll be an extremely small possibility a Gigantamax Pokémon will appear instead of a Dynamax Pokémon, but during an event, certain Gigantamax Pokémon are more likely to appear. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/gigantamax-snorlax-and-gigantamax-duraludon-in-january-max-raid-battles/