How do you activate the Arceus event?

It's not clear what you mean by "activate the Arceus event." Please provide more context.

For example, are you referring to:

* A specific Pokemon game: Are you talking about a specific Pokemon game like Pokemon Brilliant Diamond & Shining Pearl, Pokemon Legends: Arceus, or another game?

* An in-game event: Did you see a notification or notice about an event involving Arceus in a specific game?

* A real-life event: Are you talking about a promotional event or giveaway related to Arceus?

Please tell me more so I can understand what you're asking!