In March 2011, there was an event going on at all local GameStops. It was even broadcasted on TV that an Event Celebi would be given out for a short period of time before the release of Pokemon Black and White Versions. When you go to GameStop, you would select Mystery Gift and then Receive from Wireless. This would give you the pass to receive Celebi, which could be accessed when you talk to the deliveryman in any Poke Mart. This could be done for any versions compatible for the DS; this includes Diamond, Pearl, Platinum, HeartGold, and SoulSilver. If you got Celebi in HeartGold or SoulSilver, you could access a quick cutscene where you would travel in time with Celebi.
To do this, you would need to prepare your strongest team (just in case you were faced with a strong opponent). Then, go to the Ilex Forest with your Celebi as your lead Pokemon (it has to be walking behind you). When you got to the house-thing close to the entrance, your friend Lyra would come with her Marill. She would then ask how things have been, and accidentally be teleported back in time with you (and leaving her Marill behind!). After a short break, you would find yourself back three years, and in Team Rocket’s secret headquarters before you ruined their plans. There would be a member of Team Rocket in there, and challenge you to a Pokemon battle. When you won, the radio would sound out, calling for Team Rocket members. Celebi would then transport you to a different place. You would then find your rival battling Giovanni, head of Team Rocket, and win.
You can hear the conversation after the battle, which somehow makes your rival confused and leads him to run away. After you survey the area around (and check for items, even though there aren’t any), you can go challenge Giovanni. He will then tell you three years ago, a kid ruined his plans (that was in Goldenrod Radio Tower), and won’t plan to lose again.
FireRed and LeafGreen Versions have you battle Giovanni as the last Gym Leader. This time, since he isn’t here in Johto, you will have no idea what his Pokemon are. But if you have experience from previous versions, you can conclude two of his Pokemon are Nidoking and Nidoqueen, which are correct. This is a full six-on-six Pokemon battle, and since your Celebi is sent out first, you may have to switch it out (even though it is level 50). After you won the battle, he would have his team flee the scene, and of course, tell you that Team Rocket still lives on. Celebi would then teleport you and Lyra back to Ilex Forest. Lyra would then say farewell to you after finding her Marill at the same spot.
The Celebi you get here is meant to be traded to Black and White Versions, which, along with the Event Entei, Raikou, and Suicune, can get you Zorua and Zoroark from Generation V. Be careful, though, because you can’t transfer Celebi back once it’s in Black and White version.