Easily the most complicated set mechanic, Magic now has a day/night cycle that will affect the battlefield. Actions (or inaction) during each turn, along with card effects, can determine whether the battlefield is experiencing day or night, and when the time shifts, cards bearing Daybound or Nightbound creatures are forced to transform. Some werewolves in the set will have both of these keywords on opposite sides of the card and will have their transformation bound to whether the sun is shining or the moon has risen. Arlinn Kord, the werewolf Planeswalker seen above, is subject to this very transformation condition.
The game rules will only start caring about the time of day if a Day/Nightbound card is played and only affects cards where the time of day is conditional. Here’s how the time shifts if a creature or spell isn’t specifically changing the time of day:
When it’s daytime: if a player plays no spells during their turn, it becomes night next turn
When it’s nighttime: if a player casts at least two spells during their turn, it becomes day next turn
It’s a cool way of realizing the werewolf fantasy but keeping track of this while playing in paper will be a challenge. I foresee myself missing many time of day triggers by forgetting how many spells someone did or didn’t play during their turn. It’s a lot to keep track of and might be the most digital-centric mechanic we’ve seen from a Standard-legal set.