Pokémon Bank, the cloud service that lets you store and retrieve Pokémon for Pokémon X and Pokémon Y, will be available on December 27th in North America. If you download the Pokémon Bank app prior to September 30th, 2014, you’ll get a bonus Pokémon along with it—Celebi.
The bonus Celebi will come with the move Hold Back, which is a restrained attack that cannot be learnt through normal gameplay. The move leaves its victim with at least 1HP, making it a useful tool while capturing wild Pokémon.
You’ll need to use the Pokémon Link feature in Pokémon X and Pokémon Y to receive Celebi after you download Pokémon Bank. Once you’ve received Celebi within Pokémon Bank, selecting Pokémon Link will send it to your PC box in the game.
More information on Pokémon Bank can be found here. The service requires a subscription that costs $4.99 per-year, but downloading before January 31st, 2014 will get you a month-long free trial.