Zombie Events are just that: Events with TONS of Zombies. The first we'll talk about is the Zombie Waves event. Zombie Wave Events CAN happen once every 3 In-Game days. The rewards are little compared to Zombie Slayer. All this Event will give is masses of EXP used to enchant with. Once one starts there should be a message from a god, or the Terminal stating that the attack has begun. The goal is to survive until daytime. The Zombies in this event will NOT appear in towns, but appear near them, or hoards out in the wild.
The second Zombie Event is: Zombie Slayer. This will be announced by the words: Get the most Rotten Flesh! Soon after this is said, hoard after hoard will keep coming, non-stop until the sun rises. Yet again you must survive, but the more Rotten Flesh gained the better your reward will be. This Event can be ignored, if you stay inside a building or enclosed area.
The Village VS. Zombies Event is hard. You must ensure that ALL the Villagers stay alive, sometimes with the help of an Iron Golem or two. The rewards from this are usually Books of Time, a rare Book of Souls or Heros, and maybe a Book of Space if you're lucky. Only one wave comes during this Event, and the hoard must die before sunrise, or no one gets a reward. This Event targets a single Village, so it's easy to locate and destroy.