The official Pokémon website has announced that Gigantamax versions of Charizard, Garbodor, Copperajah and Duraludon will appear more frequently in Max Raid Battles until April 27th, 2020, at 4:59 p.m. PDT in Pokémon Sword and Pokémon Shield.

Gigantamax Charizard, Garbodor, Copperajah, and Duraludon Appear in Max Raid Battles

Journey into the Wild Area to catch these powerful arriving Pokémon.

Get ready for a Gigantamax take-over in Pokémon Sword and Pokémon Shield, as Gigantamax versions of Charizard, Garbodor, Copperajah, and Duraludon will appear more frequently in Max Raid Battles until April 27, 2020, at 4:59 p.m. PDT. Head into the Wild Area and begin your search for these elusive Gigantamax Pokémon.

To challenge these Pokémon in a Max Raid Battle, you'll need to connect your Nintendo Switch system to the internet. Then, whenever you see a pillar of light shining from a Pokémon Den in the Wild Area, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle with a Dynamax Pokémon. While Gigantamax Pokémon are appearing more frequently in Max Raid Battles, you'll sometimes encounter one of them instead of a Dynamax Pokémon. Outside of these special time periods, encountering a Gigantamax Pokémon is extremely rare.

Source: https://www.pokemon.com/us/pokemon-news/gigantamax-charizard-garbodor-copperajah-and-duraludon-appear-in-max-raid-battles/