you need the celestial mirror. which you get when you get and defeat every aeon in the secret temple near the calm lands. Well, actually you get the cloudy mirror or something and then you must find a way to charge it up. then you can open those boxes.

For more information, might I recommend IGN.com?

Getting the celestial weapons is kinda neat. I mean, it's kind of a kill joy to watch Lulu caste Ultima and then not have it hurt very much.