Here's a brief explanation of how to get the celestial mirror.

First go to Remiem Temple and race against the chocobo. Open 3 chests and win to get the Cloudy Mirror. When acquired, go to Macalania Forest - South and you'll see a woman and her child waiting for their father. Then go to the campsite there and talk to him. He'll run back to them. Then the child is gone. He's on the shining road, speak to him and he open the path to the large thing. There you can upgrade the Cloudy Mirror to the Celestial Mirror. Then you can get the celestial weapons.