Get Chocobo in Calm Lands and reached Remiem Temple. Win the race and receive Cloudy Mirror (you must search for a sphere around the temple but not inside!). Go to Macalania entrance, then meet with this family which lost their father, search their father at the place where Rikku sleep. Then meet with this family, they say they lost their son, go back a little until you saw a shining way, follow the path up and found their son. Use your Cloudy Mirror and it will become Celestial Mirror, you can now open a chest containing Celestial Weapon.