Instructions
Check the Official Pokemon website's "News and Events" page or the Serebii Event Database for information on upcoming Celebi give-away events. On the Serebii Events Database, select "ûCelebi" from one of the drop-down menus. A list of past, present and upcoming Celebi give-away events will appear.
Take your Nintendo DS and copy of "Diamond" to the "Pokemon" Celebi give-away event. Players will receive a Level 50 Celebi.
Turn on the Nintendo system and travel to Jubilife City's Global Trade Station. Speak to the receptionist, select "Trade" and choose "Yes" twice. Select "Deposit Pokemon" and pick a Pokemon you would be willing to trade in exchange for Celebi. Next, select "Celebi" from a list of desired Pokemon. Travel to the Global Trade Station each time you play to see if another player has accepted the trade.
Trading from GBA Games
Insert a Game Boy Advance "Pokemon" game containing Celebi, such as "Ruby" or "Sapphire," into the Game Boy Advance slot of the Nintendo DS system. Insert "Diamond" into the Nintendo DS slot of the system. Turn on the system.
Select "Migrate From..." and choose the Game Boy Advance "Pokemon" game from the list. Select "Celebi" to migrate. This action is permanent, and players will not be able to transfer Celebi back to a Game Boy Advance game.
Go to Pal Park, located along Route 221. Enter Pal Park and travel to the Forest region of the park. Continue to roam here until you find Celebi in a random encounter. Throw one of the Park Balls to capture Celebi and permanently transfer it to "Diamond."