By and large there's no FAR (other than perhaps reckless and dangerous)
that prohibits landing at closed airports. ATC will even advise you
the airport is closed, but if you choose to do so you operate at your
own risk.

The ones that will get you in trouble is if there is some special flight
restriction that closes the airport, but if the Mayor paints X's on it
or a landowner declares it closed, the FAA is not involved.