Hound Tamer // Untamed Pup — ruling 1

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.

Hound Tamer // Untamed Pup — ruling 2

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

Hound Tamer // Untamed Pup — ruling 3

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

Hound Tamer // Untamed Pup — ruling 4

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

Hound Tamer // Untamed Pup — ruling 5

If it's neither day nor night, and a creature with daybound and a creature with nightbound somehow appear on the battlefield at the same time, it becomes day. The creature with nightbound will transform.

Hound Tamer // Untamed Pup — ruling 6

If you cast a spell with daybound during night, that spell will be front face up (that is, daybound face up) on the stack. However, it will enter the battlefield with its back face up (that is, with its nightbound face up). It won't enter with its daybound face up and then transform.

Hound Tamer // Untamed Pup — ruling 7

Permanents with daybound and nightbound can't transform via any means other than their daybound and nightbound abilities. Notably, older cards such as Moonmist that instruct a player to transform permanents don't affect permanents with daybound or nightbound.

Hound Tamer // Untamed Pup — ruling 8

If it is night, permanents with daybound that enter the battlefield without being cast will enter with their nightbound faces up.

Hound Tamer // Untamed Pup : règles, fonctionnement et interactions (MTG)

Hound Tamer // Untamed Pup 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

Trample : Put a +1/+1 counter on target creature. Daybound (If a player casts no spells during their own turn, it becomes night next turn.) Trample Other Wolves and Werewolves you control have trample. : Put a +1/+1 counter on target creature. Nightbound (If a player casts at least two spells during their own turn, it becomes day next turn.)

How it works : Hound Tamer // Untamed Pup

Original English text — Wizards of the Coast / Scryfall

#1September 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.
#2September 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.
#3September 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.
#4September 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.
#5September 24, 2021· Wizards of the Coast
If it's neither day nor night, and a creature with daybound and a creature with nightbound somehow appear on the battlefield at the same time, it becomes day. The creature with nightbound will transform.
No other card is named in this ruling.
#6September 24, 2021· Wizards of the Coast
If you cast a spell with daybound during night, that spell will be front face up (that is, daybound face up) on the stack. However, it will enter the battlefield with its back face up (that is, with its nightbound face up). It won't enter with its daybound face up and then transform.
No other card is named in this ruling.
#7September 24, 2021· Wizards of the Coast
Permanents with daybound and nightbound can't transform via any means other than their daybound and nightbound abilities. Notably, older cards such as Moonmist that instruct a player to transform permanents don't affect permanents with daybound or nightbound.
No other card is named in this ruling.
#8September 24, 2021· Wizards of the Coast
If it is night, permanents with daybound that enter the battlefield without being cast will enter with their nightbound faces up.
No other card is named in this ruling.
#9September 24, 2021· Wizards of the Coast
Double-faced permanents with daybound transform to their nightbound faces as it becomes night. Similarly, double-faced permanents with nightbound transform to their daybound faces as it becomes day. This happens immediately and is not a state-based action. It happens any time it becomes day or night, not just during the untap step.
No other card is named in this ruling.