Werewhat

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.

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).

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.

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.

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.

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.