Rule 731.2 · Day and Night
As the second part of the untap step, the game checks the previous turn to see if the game’s day/night designation should change. See rule 502, “Untap Step.”
Subrules
If it’s day and the previous turn’s active player didn’t cast any spells during that turn, it becomes night. Multiplayer games using the shared team turns option (see rule 805) use a modified rule: if it’s day and no player from the previous turn’s active team cast a spell during that turn, it becomes night.
If it’s night, and previous turn’s active player cast two or more spells during the previous turn, it becomes day. Multiplayer games using the shared team turns option (see rule 805) use a modified rule: if it’s night and any player from the previous turn’s active team cast two or more spells during that turn, it becomes day.
If it’s neither day nor night, this check doesn’t happen and it remains neither.