This wild area event held by Sword and Shield. It made Magikarp the focus of the event, with a high chance to encounter one shiny, to celebrate New Years 2020. The Shiny Magikarp is a 8% guaranteed spawn, across all tiers.
While Magikarp crashed the #2 event, it merely shifted the spawns. As such, Gigantamax Snorlax is still available, just at a lower rate.
For base game Pokémon Sword and Shield, without this event, there is no den that will enable players to encounter a Gigantamax Snorlax.
As per the previous events, Drednaw and Sandaconda were Sword exclusive raids, and Corviknight and Centiskorch were Shield exclusive raids. Butterfree and Snorlax can be obtained regardless of game.
Though it is important to note, host can include players of the opposite version to join in the raids.
This event ran globally from December 31st, 2019 to January 3rd, 2020.
When this event ended, it shifted back to Wild Area Event #2 (Index 07 in this case).
To import these files into your save, use PKHeX's block importing function. Our tutorial should guide you through it.
Our first post of this page would contain some data relevant to the importing stage.
There's info and some common questions regarding Wild Area Events here.