There’s a chance for the moon to be Blue, when it’s blue you’re going to want to go to Lost Islands (Talk to the man next to the ship in Port Decca, if you don’t have this ticket yet, you buy it for 1,000 Pokedollars in Decca Agency (Yellow building).) 

Once you’re in Lost Islands, just walk to any grass patch or even surf and the encounters will be different with a small chance you could either get Cosmog or Regieleki. And yes, you can evolve Cosmog to either Lunala or Solgaleo. Cosmog evolves to Cosmoem at Level 43 and Cosmoem evolves to either Lunala or Solgaleo at Level 53. To get Lunala, evolve Cosmoem at night and to get Solgaleo, evolve Cosmoem during the day.