These are likely endings (not that players ever do anything likely...)
The heros figure out what is going on before the great festival. They talk Midnight Mere and Summer Breeze into cooperating, by sacrificing the Spells of Peace & Isolation, therefor restoring the day and the night (even if they did not realize this would happen). (Fate: Recovery)
The heros figure out what is going on and get Summer Breeze to sacrifice isolation to restore the day, but not until after Midnight Mere has sacrificed River Reed/Innocence to 'gain' war and restore the night. One possible variation is that if the heros figure out what is going on before the ritual, and after explaining the situation to River, she willingly allows herself to be sacrificed for the betterment of the realm - trusting the heros to make sure Summer completes the other half, of course. (Fate: Recovery.)
The heros never figure out what is going on. The ritual goes off, and the realm is plunged into eternal night, still cut off but able to war within itself. The heros, are, of course, stuck here, so they will hopefully figure it out eventually. (Unless of course Summer Breeze got killed somehow...) (Fate: Corruption.)
The heros decide that the worshipers of Shimmer are evil, and end up putting the followers of Shine in the leadership position, in the process ending the Spell of Isolation and making it eternal day.... They are free to go, leaving behind a very interesting realm, which will probably become famous not only for eternal day, but as a sanctum for those fleeing from war. (Fate: Corruption.)
The heros don't figure anything out, but stop the ritual from taking place on principal. Things stay pretty much the same, although they may affect some social changes in their (life)long stay. (Fate: no change)