Reminders send a set number of days ahead of the patient's scheduled appointment, at 10:00 am local time. The exact number of days depends on the setting used when configuring appointment reminders: the recommended default setting is one day ahead of confirmed appointments, and three days ahead of unconfirmed appointments.
For example, at 10:00 am on Monday, a query checks for any confirmed appointments on Tuesday and any unconfirmed appointments on Thursday, and sends the reminders. In particular, if on Monday after 10:00 am, a confirmed appointment is added for Tuesday, no reminder will be sent, because the event was created too late. In this case, the user creating the event will want to send the appointment invitation when prompted. Similarly, if on Monday after 10:00 am you add an unconfirmed appointment for Thursday, its reminder will not send unless the appointment gets confirmed by Wednesday at 10:00 am, in which case the reminder will send on Wednesday at 10:00 am.
A reminder sends via email if the contact has an email address on file and allows emails; otherwise, if the contact has a mobile phone number on file and allows texts, it will send via SMS.