The latest Nintendo Direct has just finished airing and it was announced that players who access Pokémon Bank before September 30th 2014 will receive a special Celebi. You can transfer Celebi to Pokémon X/Y using the Pokémon Link feature. Even if you store Pokémon from multiple games, only one Celebi is available per copy of Pokémon Bank. More information about Celebi’s moveset, held item, etc. have not been detailed yet so keep checking back!
Celebi’s details have been revealed on the official japanese website. It will be at Level 10 with the moves Recover, Heal Bell, Safeguard and Hold Back. Hold Back is a new move that has the same effect as False Swipe, always leaving the target with 1 HP.