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If it's neither day nor night, it becomes day as Celestus Sanctifier enters the battlefield.
Whenever day becomes night or night becomes day, look at the top two cards of your library. Put one of them into your graveyard.
Before a player untaps their permanents during the untap step, the game checks to see if the day/night designation should change.
Day and night are designations that the game itself can have. The game starts as neither. Once the game becomes day (or less commonly, night), the game will be exactly one of them—day or night—going back and forth for the rest of the game.
For more information about Day and Night and the Daybound and Nightbound abilities, please see the Innistrad: Midnight Hunt Mechanics atricle (https://magic.wizards.com/en/articles/archive/feature/innistrad-midnight-hunt-mechanics-2021-09-02).
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.
If it is day, and the active player of the previous turn cast no spells during their turn, it becomes night.
If it is night, and the active player of the previous turn cast two or more spells during their turn, it becomes day.
The card you don't put into your graveyard stays on top of your library.
