Is my California eviction response deadline ten business days or ten court days?

Court days, and the distinction is bigger than it sounds. Court days skip judicial holidays in addition to weekends, which business days don't necessarily account for the same way, depending on how your workplace defines them.

That one-day gap only surfaces in months containing a court holiday, a state observance, Presidents' Day, whatever happens to fall in your particular window. That single extra day of difference has been the entire reason a defendant filed late and lost a case they otherwise had a real shot at. Confirm the actual court calendar rather than counting on your fingers.