If it's day as a turn begins, and the previous turn's active player didn't cast a spell last turn, it becomes night. Similarly, if it's night as a turn begins, if the previous turn's active player cast two or more spells last turn, it becomes day.

Coven is an ability word that's used to highlight abilities that get better if you control three or more creatures with different powers. This is where you may have a moment where you think that "powers" means "abilities," but no, "powers" is just the number before the slash opposite toughness.