The trigger to using the stones is the shrine on Meadow Island. You can't collect any Sun Stones until Spring 10 or the first sunny day thereafter, when the entire island population hears a strange voice coming from the Meadow Island. Only you can see Agete the red Harvest Sprite, keeper of the Sun Stone shrine. Agete explains that the rest of the islands sunk and asks that you bring him Sun Stones so that they can return to the surface. You can resurrect the islands in any order you want. You just have to collect the necessary Sun Stones: