Oritsuru » Final Fantasy X » Butterfly Catcher Sidequest
In order to get the Saturn Sigil which is used to unlock Kimahri's legenday weapons, you'll need to head back to the Macalania Woods and complete the butterfly catcher sidequest. If you haven't already, look for someone who will explain what the mini-game is about. Afterwards, find the closest butterfly and touch it. A timer should come up and you'll have to catch 7 of the blue butterflies before the time runs out! Make sure to avoid the red butterflies. The red ones will remove one second off of your clock and thrust you into battle.
If you collect 7, a chest will appear. With a prize inside. As you the game goes along, time will be taken off and more difficulities will arise, so beware!
Once you have the airship and can return, you should notice there are more red butterflies. Successfully complete both areas in the sidequest and you'll be able to get the Saturn Sigil.
Yes, there are two areas to play this mini-game and you have to complete both in order to get the key item. The two areas are located in the Central and North areas of the woods.