Rule 502.2 · Untap Step
Second, if it’s day and the previous turn’s active player didn’t cast any spells during that turn, it becomes night. If it’s night and the previous turn’s active player 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. This turn-based action doesn’t use the stack. See rule 731, “Day and Night.”
Subrules
Multiplayer games using the shared team turns option use a modified rule. If it’s day and no player on the previous turn’s active team cast a spell during that turn, it becomes night. If it’s night and any player on the previous turn’s active team cast two or more spells during the previous turn, it becomes day. If it’s neither day nor night, this check doesn’t happen and it remains neither. This turn-based action doesn’t use the stack.