The Celestus — ruling 1

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

The Celestus — ruling 2

If you choose to draw a card with The Celestus but some effect replaces that card draw with something else, you will still have to discard a card.

The Celestus — ruling 3

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

The Celestus — ruling 4

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.

The Celestus — ruling 5

If it is day, and the active player of the previous turn cast no spells during their turn, it becomes night.

The Celestus — ruling 6

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.

The Celestus : règles, fonctionnement et interactions (MTG)

The Celestus 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's neither day nor night, it becomes day as The Celestus enters. : Add one mana of any color. , : If it's night, it becomes day. Otherwise, it becomes night. Activate only as a sorcery. Whenever day becomes night or night becomes day, you gain 1 life. You may draw a card. If you do, discard a card.

How it works : The Celestus

Original English text — Wizards of the Coast / Scryfall

#1September 24, 2021· Wizards of the Coast
If it is night, and the active player of the previous turn cast two or more spells during their turn, it becomes day.
No other card is named in this ruling.
#2September 24, 2021· Wizards of the Coast
If you choose to draw a card with The Celestus but some effect replaces that card draw with something else, you will still have to discard a card.
No other card is named in this ruling.
#3September 24, 2021· Wizards of the Coast
Before a player untaps their permanents during the untap step, the game checks to see if the day/night designation should change.
No other card is named in this ruling.
#4September 24, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#5September 24, 2021· Wizards of the Coast
If it is day, and the active player of the previous turn cast no spells during their turn, it becomes night.
No other card is named in this ruling.
#6September 24, 2021· Wizards of the Coast
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.
No other card is named in this ruling.