The official Pokémon website has announced that Max Raid Battles featuring Gigantamax Corviknight and Gigantamax Hatterene will take place in Pokémon Sword and Shield from March 1st, 2021 to April 1st, 2021. Flying-type and Fairy-type Pokémon will have an increased chance of appearing as well.

Pokémon Sword and Pokémon Shield Max Raid Battles Feature Gigantamax Corviknight and Gigantamax Hatterene

Glide into Galar to face Fairy-type and Flying-type Pokémon throughout March.

Soar along the path to Pokémon Trainer greatness by participating in Max Raid Battles featuring Flying-type and Fairy-type Pokémon. From March 1, 2021, to April 1, 2021, Flying-type and Fairy-type Pokémon will have an increased chance of appearing in Pokémon Sword and Pokémon Shield Max Raid Battles. During this time, Gigantamax Corviknight and Gigantamax Hatterene will also be appearing, providing Trainers with the opportunity to catch Raven Pokémon and Silent Pokémon with the Gigantamax Factor. Once caught, these Pokémon can provide their Trainers with the ability to unleash the forces of their unique G-Max Moves, G-Max Wind Rage and G-Max Smite.

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/pokemon-sword-and-pokemon-shield-max-raid-battles-feature-gigantamax-corviknight-and-gigantamax-hatterene/