First go to vermillion talk to lt surge then go to saffron and talk to the copycat girl she'll mention something about johto's il ex forest then take a pikachu from the yellow forest on the poke walker there a strange light will appear and celebi is there catch it it's at lv 45 go back there the next day and you'll see ethan or lyra with their ma rill you guys talk for a while then celebi starts to glow and you back in time hope this helps
You can't get Celebi in soulsilver but I think they're going to make an event with Celebi where you get Celebi and you travel back in time so see your rivals past. Hope this helped!