Like @Noodlesssss said, it has potential. It's an alright idea but it is so poorly executed. Most notably on Clouds. If warden called a zombie day on Clouds you might as well just watch YouTube while everyone camps armory secret.
One immediate change is the time limit. It is unnecessarily long, nobody finds it enjoyable to try and get into secrets for almost 5 minutes while being M4 spammed into the air. Not to mention some secrets are literally impossible to do as a zombie due to the gravity change you receive.
It can be fun if humans participate in ways that aren't just hiding in one ways. Even on maps like Obama, it's fun as hell to camp on top of climb and just play a ZE server temporarily as Zombies try and strafe around the sprays. But currently it doesn't feel like a ZE gamemode, it just feels like a cancerous adaptation of HnS.
Also most of the time only one person starts the round as a Mother Zombie. That shit feels HORRIBLE. Most of the people you can infect are just AFK in cells.
Unlike ZE, there is no end goal for the humans except for the timer. I have no clue how you would implement an ending on any JB maps but the timer just isn't sufficient.
Another idea to help the day is give zombies special items like some ZE maps have, i.e a vacuum to suck humans out of secrets.
I'm not sure if the idea of a zombie day is a lost cause, but temporarily it needs to be taken out of the !days menu and we need a decent substitute. You know it's awful when Jihad day is more exciting.