When you have to give 24 hours notice for a doctor's appt?

My daughter has a dentist apointment Monday at 4 pm. She just found out on late last night (Friday) that she has the opportunity to go to New York and see a Broadway play with her best friend. She tried calling the office at 6 pm last night, but they were closed. She called first thing this morning hoping that they had Saturday hours, but they don't. She left a message on their answering machine cancelling the appt and said she would call on Tuesday to reschedule. Since they will not get the message until Monday morning, will they charge her for not giving 24 hours notice?