Do you know if it's ok to stay overnight in a closed campsite?
What part of closed do you not understand?
You mean private property not open for business? Why would it be ok unless you have been explicitly allowed?
I see, just asking if it's tolerated, thanks
Most campsites here are owned by individuals or families so essentially you'd be trespassing.
Additionally, you may damage wet grass with vehicle wheels.
Having said that, I've stayed at a few closed campsites in the UK and France when cycle-touring or backpacking in the off-season but have asked where possible.