If you exile a creature card to Werewhat’s enters ability, Werewhat will be a transforming double-faced card for as long as it remains on the battlefield. Then if it leaves the battlefield to another zone, the two cards would separate and both be put into the new zone.

If you exile a card to Werewhat’s enters ability that is already a double-faced card, you may choose either face of the card to serve as the back face of Werewhat. The face you didn’t choose would be treated as nonexistent for as long as Werewhat remains on the battlefield. If your choice somehow ends up with an instant or sorcery card as the back face of Werewhat, Werewhat would be unable to transform away from its daybound side, so it would remain daybound even if it’s night.

Any time a player controls a permanent with daybound, if it’s neither day nor night, it becomes day. Less commonly, any time a player controls a permanent with nightbound, if it’s neither day nor night and there are no permanents with daybound on the battlefield, it becomes day.

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 (barring someone killing the sun with Naturalize 2, I suppose).

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

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.

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.

If you cast a double-faced 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 the battlefield with its daybound face up and then transform.

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

Double-faced 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 won’t affect permanents with daybound or nightbound.

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. If it’s double faced, the creature with nightbound will transform.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.