https://www.polygon.com/2018/8/16/17698862/pokemon-go-celebi-research-task-how-to-get

Pokémon Go Fest 2018 attendees already have their own Celebi, the Johto region’s exclusive Mythical Pokémon. Players who weren’t able to attend this year’s live event will have their chance to capture it starting Aug. 20, when Niantic releases a special Research task that offers Celebi as a reward.

The developer is vague about what that challenge will include, saying only that it will require players to ... test their skills. Which, that’s nothing new. But if it’s anything like what those of us at Pokémon Go Fest had to do, expect it to take a good chunk of time to complete everything necessary to earn the right to capture Celebi.