The official Pokémon website has announced that Pikachu, Raichu, and Pichu will be in Max Raid Battles from August 6th, 2020 until August 16th, 2020 in Pokémon Sword and Pokémon Shield.

Discover Pikachu, Raichu, and Pichu in Max Raid Battles in Pokémon Sword and Pokémon Shield

Start searching those Pokémon Dens! The Pikachu Mass Appearance event runs through August 16.

Your Pokémon collection in Pokémon Sword and Pokémon Shield is about to get a lot more yellow! A Pikachu Mass Appearance event is happening now until August 16 at 4:59 p.m. PDT. During this event, Pikachu, Raichu, Alolan Raichu, and Pichu will appear more frequently in Max Raid Battles. Lucky players may even encounter a Shiny Pikachu.

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. During this event, Pikachu are more likely to appear as Gigantamax Pokémon, so keep checking dens.

Good luck battling and catching the Pikachu family in this Pikachu Mass Appearance until August 16.

Source: https://www.pokemon.com/us/pokemon-news/discover-pikachu-raichu-and-pichu-in-max-raid-battles-in-pokemon-sword-and-pokemon-shield/