That doesn't sound like a good planned move to be fair. I mean this is a casino and casinos are usually open 24/7/365, I don't understand how they couldn't just find anyone that could fill that shift for the day or for the duration of the "vacation" or whatever the reason is for it.
If they announced it officially, with time gaps for their users to withdraw money, and if they will come back to fully normal operations after the holidays, everything is fine, IMO. As far as I know of, FortuneJack has built up their highly reputation here. Most of gamblers love them.
I guess their visionary is: If they allow users to bet and withdraw money during holidays, when their support team are off or lack of human resources for support tasks, when withdrawal issues come from customers, how they handle support tickets. Then, their users will fall into paranoid, that is worse.
Exchanges can fall into troubles sometimes but people still use them. I doubt that all users will abandon FortuneJack or whatever other reputable platforms for some kind of reasons that don't seriously damage their funds or their lives.