The official Pokémon website has announced that for the month of December 2020, Ice-type Pokémon Lapras, Weavile, and Avalugg, along with the Fire-type Pokémon Rapidash, Turtonator, and Centiskorch will appear more frequently in Max Raid Battles. Plus, Gigantamax Lapras and Gigantamax Centiskorch will appear more often.

Gigantamax Lapras Chills and Gigantamax Centiskorch Cooks in Max Raid Battles in December

You’ll find Ice-type and Fire-type Pokémon to a greater degree in Galar’s dens through the end of the year.

Whether you want to stay cool with Ice-type Pokémon or heat up the action with Fire-type Pokémon, December is your month to explore Pokémon Dens in Pokémon Sword and Pokémon Shield! Through December 31 at 3:59 p.m. PST, Pokémon Dens all over the Galar region will feature the Ice-type Pokémon Lapras, Weavile, and Avalugg, along with the Fire-type Pokémon Rapidash, Turtonator, and Centiskorch more frequently in Max Raid Battles. Plus, Gigantamax Lapras and Gigantamax Centiskorch will appear more often. Note that this event does not affect the Pokémon encountered in Dynamax Adventures.

Source: https://www.pokemon.com/uk/pokemon-news/gigantamax-lapras-chills-and-gigantamax-centiskorch-cooks-in-max-raid-battles-in-december/