Kessig Naturalist // Lord of the Ulvenwald

If it's neither day nor night, and a creature with daybound and a creature with nightbound somehow appear on the battlefield at the same time, it becomes day. The creature with nightbound will transform.

Day and night are designations that the game itself can have. The game starts as neither. Once the game becomes day (or less commonly, night), the game will be exactly one of them—day or night—going back and forth for the rest of the game.

Before a player untaps their permanents during the untap step, the game checks to see if the day/night designation should change.

If it is night, permanents with daybound that enter the battlefield without being cast will enter with their nightbound faces up.

If it is day, and the active player of the previous turn cast no spells during their turn, it becomes night.

Permanents with daybound and nightbound can't transform via any means other than their daybound and nightbound abilities. Notably, older cards such as Moonmist that instruct a player to transform permanents don't affect permanents with daybound or nightbound.