My T does her own schedule, and I have set appointments. She has been somewhat flexible if I have to miss a session. We re-worked the schedule this fall, and she asked if I could switch times for one session. She schedules her vacations out months in advance. She had to miss 2 sessions when her mother passed away, and she was out sick for a week, but those have been the only short notice, unplanned absences in 2.5 years.