One big part of Pokémon Sword and Shield is the ability to participate in Max Raid Battles with four other trainers, in order to take down special Dynamax Pokémon. In some rare cases though, a Gigantamax Pokémon will appear.
While the chances of encountering one of these Gigantamax Pokémon are normally incredibly low, there'll actually be regular events in the game where certain Gigantamax pocket monsters become more common during a specific period. As previously noted, the first one up for grabs is Gigantamax Butterfree.
From Friday, November 15, 2019, until early January 2020, players will be more likely to encounter Gigantamax Butterfree in Max Raid Battles.
During this same period, players who own a copy of Sword will have more regular encounters with Gigantamax Drednaw, and players who own Shield will more regularly encounter Gigantamax Corviknight.
The Gigantamax Pokémon you can encounter will change once this event ends. So keep an eye out for future updates like this one.