The Celebi event for HeartGold and SoulSilver has already came and went. It started on February 21, 2011 but it ended on March 7, 2011. There is no chance of the Celebi event download for HeartGold and SoulSilver repeating so the only way you can get the event Celebi is by trading for it.
You need to have the Celebi event. it is over. sorry
you get it by an event.
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No.
Mystery Gift.
Nope has to be the special event Celebi in HeartGold and SoulSilver
No, it is not possible to do the event in HeartGold and SoulSilver with any Celebi because the game reads what is encoded in Celebi's summary. You need a Celebi with a "Fateful Encounter" flag on it (this flag will be shown in its summary) in order to unlock the event in HeartGold and SoulSilver.
You can trade it from Diamond, Pearl, Platinum, HeartGold and SoulSilver.
I'm afraid you cannot get a Celebi in Pokemon HeartGold, unless you use a Action Replay. The only Celebi legal, I think, is the Nintendo Event that has passed. If you do have the Celebi in your GameBoy Pokemon Cartridge, then you can migrate it to Pal Park, then trade from Pearl/Diamond to HeartGold/SoulSilver.
Celebi cannot be caught in Pokemon SoulSilver. Celebi can only be obtained through an event or Action Replay cheat. Celebi cannot be found in HeartGold, either.
you will have to transfer one from Pokemon diamond, pearl,platinum,soulsilver,and heartgold