We have found that in places like Paris and Rome, there often are unscheduled closings.  It pays to call ahead, even the day before.  When they give reasons, they point to work stoppages, employee absences or budget cuts.  I heard a sad story about a valuable day in Paris being lost to on a daytrip by bus and rail to a palace outside the city that was closed.  All I can say is, try to be philosphical and roll with the punches, but call ahead if at all possible.