Brimstone Vandal

If it is night, and the active player of the previous turn cast two or more spells during their turn, it becomes day.

If a triggered ability triggers whenever day becomes night or night becomes day, it only triggers if it was already either day/night and becomes the other. It does not trigger when the game becomes day or night for the first time.

Before a player untaps their permanents during the untap step, the game checks to see if the day/night designation should change.