If it becomes day or night, or if a daybound permanents enters the battlefield, track day/night for the rest of the game.
As it becomes day, transform all nightbound permanents.
If a player casts no spells during their own turn, it becomes night next turn.
---
As it becomes night, transform all daytbound permanents.
Permanents enter the battlefield nightbound.
If a player casts at least two spells during their own turn, it becomes day next turn.