HGSS details box on the front page of Serebii's news page(
Link), it appears you need to box your entire party, save Arceus, before the event fully activates and completes. It's near the end of the spoiler box.
... Wait, did you think I meant the event to get Arceus? I meant the one requiring Arceus to get the Level 1 Dragons...
/EDIT: Apparently all the berry locations are Apricorns now, according to PokeBeach, and one of the staff on the Serebii Forums say it's quicker to save the game than Plat, which is good.
