Day and night are game designations that can change according to how many spells the active player cast during the previous turn. Permanents with daybound and nightbound transform automatically as the game moves between the two states.
Reckless Stormseeker becomes the considerably hairier Storm-Charged Slasher at night, while Moonrager's Slash becomes cheaper. Cards such as Tovolar, Dire Overlord and The Celestus give decks additional ways to manipulate the cycle.
