To do this event, you need to create a mock event Celebi. The actual event Celebi has not been released yet, so this is the only way. I used PokéSav to create this one. It creates a level 50 Celebi that was "received" at "Pokémon Movie '10" (which hasn't actually happened yet and probably won't be about Celebi anyways). Press L+R to make it appear in box 15, slot 1 (HG/SS only!).

94000130 FCFF0000
62110DC0 00000000
B2110DC0 00000000
E001D710 00000088
27CAA37F E8D00000
698E283B 7A13AB93
4A722A95 A1A91EB7
E1E59503 D809955C
4016398C F05BB11C
7438E884 48C50719
A76B5C8B 8BD58F2C
38209904 0656681D
6F2A69BB F52FC00F
99066787 83F91AC4
5264D6D8 8FF522C9
70D1BAA9 966D8110
EE3869A7 9F5C5D81
9EE9E2D7 C02578FB
2749AD5E 17B7BEF5
FF6877DF 8B612DEA
71397642 FE033908
D2000000 00000000

Kotone will confront you before you see a flash of light on the screen.

You go back in time to find your rival talking to a man wearing a trench coat and a hat. After their little chat, he'll confront you before running off to the side. Celebi will transport you again, this in a cave where that trench coat guy is. He'll talk about Team Rocket and whatnot before challenging you to a battle... and, hey, it's Giovanni! His team is pathetically weak, though, so I just used my in-game team (which sucks pretty bad).

After the battle, you'll go back to your own time. (less info)