Legendary Pokémon

I have heard that if you have a Celebi taken from a special event you can activate an extra event that took place in the past of Giovanni (spoiler alert):
http://www.serebii.net/heartgoldsoulsilver/celebi.shtml

My question is: which exactly special event Celebi you will need in order to activate this event? I have tried many different Celebi (either legally or ilegally gotten) but none of them did the trick. Any help please?

That was one specific Celebi that was given away prior to the Zoroark movie in Japan and in certain events in US and Europe. Here in Greece it was given away by Plaisio about a month ago if you had preordered Pokémon Black and White and went to their shops with the receipt of the order. Well, that would be the case normally but if you went there with your DS, even if you hadn't preordered you would get it anyways

If you'd like one I have like 3 of'em.

It's the one mentioned over here: http://www.legendarypokemon.net/2011/01/11/Greek/celebi
There might be yet another chance to obtain it, right here in Greece in the near future...

Well, well! And in the official site of Nintendo.gr it was mentioned that only by preorder Pokemon Black/White from Plaisio you can obtain it. Talking about @#$%^&* lies. Thanks heaven to Legendary Pokemon for opening my eyes.

Adk I'd like a celebi I'll send you a pm.