Shady Traveler

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

If it is night, and the active player of the previous turn cast two or more spells during their turn, it becomes day.

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.