You can't without cheating, so dont listen to anything that says erase your data.
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No you cannot catch Celebi in Pokemon Emerald.
you can't
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I got my Celebi for Emerald from the Pokemon 10th Anniversary. You can only get it through Pokemon events, or cheat devices such as Action Replay.
Here... Master Code (must be on)D8BAE4D94864DCE5A86CDBA519BA49B3061221968FE9826065DA389A09D8882C Celebi252141700AB256A24AEC27E8A5FF15406B012323530CEE0096BB89299B4E5B77