[imgdq3]http://www.serebii.net/bank/celebi.jpg[/imgdq3]
"When it was first released at the start of this year, Pokémon Bank came with a special gift for everyone who opened up their Pokémon Bank, either in the trial or afterwards. This give was a special Level 10 Celebi with the special move, Hold Back. This Celebi is only to be distributed for 10 more days, with the event ending on [bdq3]September 30th[/bdq3]. From that point on, you'll be unable to receive it, so be sure to get it while you can."
[imgdq3]http://media.giphy.com/media/n824SIfb1oM6s/giphy.gif[/imgdq3]
Method:
If you deposit Pokémon onto Pokémon Bank, or withdraw them, you will get access to this special Celebi. This can be done if you have paid for Pokémon Bank or if you are in the 30 day free trial. To get it, you need to have deposited Pokémon into Bank from X & Y. Next time you load the game up, Brigette will give you Celebi.
source: [urldq3]http://www.serebii.net/[/urldq3]