Answer: Make the appointment as early as possible in the day so you can return to work when finished.
When you have an appointment that you cannot miss, it is likely that your work is going to be affected. In the case of the dentist appointment, for example, it is unlikely that the appointment can be made for after hours. Therefore, the best option you have is to have the appointment as early as possible during the day. This ensures that you are able to return to work when finished and that your work is disrupted as little as possible.