You must defeat Darkrai first. Afterwards, talk to the two Ursaring and they will tell you Luminous Spring feels different.That means that the fabric of space has returned to normal and you(The main characters)may evolve when needed. Also, Manaphy must be recruited and the Marine Resort must be unlocked. Though you can evolve any of your recruited Pokemon right after you unlock Luminous Spring.
Good luck in defeating Darkrai :)