Although events in Pokémon Go allow players a better chance of catching specific Pokémon, it primarily attracts people by giving those Pokémon new costumes to wear. This typically doesn't change much about the Pokémon itself, but it's a great way to add another level of exclusivity to events. During the 2021 Heritage Season holiday event, Glaceon can be obtained with a holiday module wearing a winter cloak. Here's how to get it.
Although Glaceon is one of the Pokémon featured during the 2021 Holiday Event, you cannot catch it in the wild and it will not appear in Field Research Meetings. Also, evolving one of your Eevees into Glaceon won't give him the party costume. Instead, you'll have to encounter Holiday Glaceon in a three-star raid. Other Pokémon you may encounter in three-star raids include Holiday Stantler, Cloyster, and Holiday Delibird. These raids are the exclusive place to get Holiday Glaceon.
When you come across Holiday Glaceon in a three-star raid, you'll want to counter it with steel or fire-type attacks. The Pokémon you will want to avoid using against it are flight, earth, grass, and most of all the dragon. Once you and everyone else in the raid take down the Glaceon's health, you all have a chance to capture it. If you're lucky, you might even get a shiny shape.