Celebi was only available in Crystal from an event, years ago when the game was released. You got a "GS Ball" apparently and it made Celebi appear in Ilex Forest.
You can't get it now in Crystal, but there may be events to get it in HG/SS.
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.