The easiest way to get Celebi in HeartGold and SoulSilver is to use the DNS Exploit, which uses fan-hosted servers to access old Mystery Gifts. A Celebi obtained through this method is legitimate and will pass hack checks. For help with setting this up, our guide is a great place to look.
This will specifically net you the Winter 2011 Celebi, which 1) lets the Giovanni event take place and 2) may be transferred to BW via the Relocator to obtain Zorua.
If you take an event celebi to the llex forest. It will take you 3 years back in time, where you discover that your rival is the son of giavonni, that is why he hates team rocket. You also get to battle giavonni.
more info here
http://www.serebii.net/heartgoldsoulsilver/celebi.shtml
If you take it to illex forest you battle ginavvi.
After getting Celebi, go to Ilex Forest. You will go back in time and find out your Rival is Giovanni's son. Then you go foward a bit to the Radio Tower takeover. You find yourself in a cave and facing Giovanni.
