(If it becomes day or night or if a daybound permanent enters the battlefield, track day/night for the rest of the game.) As it becomes day, transform all nightbound permanents. If a player casts no spells during their own turn, it becomes night next turn.
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As it becomes night, transform all daybound permanents. Permanents enter the battlefield nightbound. If a player casts at least two spells during their own turn, it becomes day next turn.