The official Pokémon website has announced that from February 22nd, 2022 until February 27, 2022, at 3:59 p.m. PST, players of Pokémon Sword and Pokémon Shield can encounter Gigantamax Venusaur, Charizard, and Blastoise in Max Raid Battles and earn Max Mushrooms.

Catch Gigantamax Venusaur, Charizard, and Blastoise in Max Raid Battles

Face the Gigantamax forms of Kanto’s fully evolved first partner Pokémon and earn Max Mushrooms in your Pokémon Sword or Pokémon Shield game.

Pokémon Day is upon us once again, and you can celebrate in Pokémon Sword and Pokémon Shield by taking on Gigantamax forms of some beloved Pokémon in Max Raid Battles. Until February 27, 2022, at 3:59 p.m. PST (that's Pokémon Day!), Gigantamax Venusaur, Gigantamax Charizard, and Gigantamax Blastoise will all be appearing in Pokémon Dens. These Pokémon can be pretty tough, but if you manage to catch or defeat them, you'll obtain a Max Mushroom for your efforts!

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/catch-gigantamax-venusaur-charizard-and-blastoise-in-max-raid-battles/