Day/Night Cycle
From King Arthur's Gold Wiki
The Day/Night Cycle was implemented before the Zombies update in order to create a suitable time for the zombies to attack. The devs decided it would also be a great addition to the other gamemodes so players could pull off ambushes, sneak attacks, etc.
Lighting
The Day/Night Cycle comes hand-in-hand with the new lighting system, added in the same update.