If you test/use Pokébank before
September 30, 2014, you'll receive a Celebi. You'll need internet access and a Nintendo ID for this.
1. If you have a cartridge of Pokémon X, insert it into your 3DS first.
2. Download and open Pokébank.
3. Activate your 30-day trial (or purchase a year’s access to Pokébank). (The trial is free.).
4. The first time, it will ask if you prefer free Battle Points or Poké Miles. Choose one of the two.
5. Deposit at least one Pokémon into Pokébank.
6. Open Pokémon X and select "Pokémon Link".
7. You will receive your Battle Points or Poké Miles. Now press Home.
8. Open Pokébank again. You can now withdraw the Pokémon you previously placed in Pokébank.
9. Start Pokémon X again and press "Pokémon Link" once more.
10. You should now find Celebi in one of your boxes.