Reminders get sent out automatically based on your Message Sequences. You do not need to trigger a reminder to send out.
You can change when reminders get sent in the Message Sequence section on Settings > Reminders. You can set the reminders to go out any number of days or hours ahead of time. You can set a reminder to be sent from 30 days to 1 hour before the appointment. Reminders will always be sent from your local time zone.
For example, if you want to send out your reminders exactly 72, 48, or 24 hours ahead of the appointment, then you can add reminders to your Message Sequence and them set them to send out reminders 3, 2, and 1 day before the appointment.
See this article for info on setting up a Message Sequence.