Day // Night

(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.