If the subpoena is ordered by the court you could be held in contempt for not appearing.  Not a good thing.  There is a fee that is supposed to come with the subpoena but in New York it is $15 so I am guessing it is nominal in Maryland too.  I would call the attorney who issued the subpoena and advise him/her that if they want you to appear that they have to pay you for taking a day off from work.  Then, if they agree, tell work you have been subpoenaed and see if you can take a day off without pay so as not to take away your vacation time.  You have nothing to lose. Good luck.