The official Pokémon website has announced that Musharna, Lilligant, Shiny Clefable, and more will appear in Max Raid Battles from Thursday, September 17th, 2020, at 5:00 p.m. PDT until Sunday, September 20th, 2020, at 4:59 p.m. PDT,

Encounter Musharna, Lilligant, Shiny Clefable, and More in Max Raid Battles

Celebrate the seasonal equinox in Pokémon Sword and Pokémon Shield with Pokémon that evolved with Moon Stones and Sun Stones.

September's seasonal equinox is almost upon us, Trainers. This day that grants us equal parts daylight and darkness is being commemorated in Pokémon Sword and Pokémon Shield with a Max Raid Battle event. From Thursday, September 17, 2020, at 5:00 p.m. PDT until Sunday, September 20, 2020, at 4:59 p.m. PDT, you will encounter raids featuring Pokémon that evolved from the use of either a Moon Stone or a Sun Stone.

During the event period, be on the lookout for more frequent raid encounters with Moon Stone–centric Pokémon—like Clefable, Wigglytuff, and Musharna—and Sun Stone–evolved Pokémon—like Bellossom, Whimsicott, and Lilligant. If you're extremely lucky, you might even encounter a Shiny Clefable.

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 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.

Source: https://www.pokemon.com/uk/pokemon-news/encounter-musharna-lilligant-shiny-clefable-and-more-in-max-raid-battles/