From February 11th, 2022 until February 20th, 2022, players of Pokémon Sword and Shield can encounter Milcery inside Pokémon Dens. These special Milcery will be able to Gigantamax after evolving into Alcremie.

Join Max Raid Battles to Catch Milcery That Can Gigantamax after Evolving

You can also collect rare Sweets when you battle the Cream Pokémon in Pokémon Sword and Pokémon Shield.

Something sweet is coming to Pokémon Sword and Pokémon Shield's Max Raid Battles. Until February 20, 2022, at 3:59 p.m. PST, the adorable Cream Pokémon may appear inside Pokémon Dens. These special Milcery will be able to Gigantamax after evolving into Alcremie. Also, when you win a Max Raid Battle against a Milcery during this time, you may receive any of the seven types of Sweets—including the rare Star Sweet and Ribbon Sweet!

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. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/join-max-raid-battles-to-catch-milcery-that-can-gigantamax-after-evolving/