Step 1: Download PokeBank
Step 2: Transfer a Pokemon from B/W or deposit a Pokemon from X & Y
Step 3: Attempt to transfer/deposit another Pokemon and a message will appear saying you've recieved the Celebi.
Step 4: On X & Y's Main Menu (starting Menu) click the "Pokemon Link" option.
Step 5: Follow the prompts and you can get your Celebi!