听力原文： You have reached the dental office of Dr. Richard Miller. If this is an emergency, please hang up and dial 800-555-3212, and an emergency operator will assist you. Our office hours are from eight thirty A.M. until four P.M. Monday through Thursday. The office is closed on Friday. If you wish to make an appointment, please call again during our regular office hours. If you wish to leave a message for Dr. Miller, please wait for the beep, then begin recording your message. If you wish to speak directly with the doctor, please leave a message and he will call you back as soon as possible. What day is Dr. Miller's office closed?