Celebi in the anime

Celebi first appeared in Pokémon 4Ever. A Pokémon hunter was after it and Celebi traveled 40 years into future to escape from him, taking the young Sammy Oak along with it accidentally. Vicious eventually captured the time traveling Pokémon in a Dark Ball and used it to destroy the forest. Ash and Sammy managed to snap Celebi out of the Iron Masked Marauder's control.

Celebi also made an appearance in Pokémon Chronicles. In Celebi and Joy, Celebi took Ritchie back in time to help stop the destruction of a Pokémon Center.

In Green Guardian, the Pokémon Ranger Solana accompanied Ash and friends in helping the time traveling Pokémon.

A Celebi appeared in the boss fantasy of A Lean Mean Team Rocket Machine.

Celebi makes a cameo appearance in the beginning of The Rise of Darkrai.

Celebi in the manga

In Pokémon Special, Pryce aimed to capture Celebi and use its time-traveling powers to save his two Lapras, which he had lost years ago prior to the start of the GSC saga.

Near the end of the RS Saga, Ruby is revealed to have a Celebi. He explains that due to the lack of information he had on it, he rarely uses Celebi at all. Celebi was used to retrieve the Red and Blue Orbs from Maxie and Archie, to prevent them from ever controlling Groudon and Kyogre again. Then, it revived Norman, Steven and Magma Admin Courtney by sending them through an altered future.

source: Bulbapedia