The Cloudy/Celestial Mirror

Go to the Remiem Temple in the Calm Lands. Talk to the Chocobo on the right hand side to race on the spiraling track under the temple. If you win, you will receive the Cloudy Mirror. To acquire the Celestial Mirror, you must take the Cloudy Mirror to Macalania Woods.

Talk to the mother and son near the Save Sphere in Macalania Woods. The mother will mention that her husband is supposed to meet her. Go to the campsite and talk to the man to the right. Tell him where his wife and son are and return to that point.

Head up the mysterious glowing path near the Save Sphere. At the intersection, head North until you see the boy standing in front of a large, spherical crystal. Talk to the boy and use the Cloudy Mirror on the crystal. This will give you the Celestial Mirror. When you want to empower a character's Celestial Weapon, you need to return to Macalania Woods and head up the Crystal Path to the large crystal. Go up to it selecting the appropriate character and the weapon itself is empowered. Go up to it a second time with the appropriate Sigil and Crest and the weapon gains its full power.