Day // Night is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

(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. 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.