To schedule an appointment that occurs “all-day” where a specific time might be irrelevant, pick a specific time to schedule all appointments while taking into consideration the desired Reminder Schedule that you will want assigned to the specific appointment.
Reminders get sent out automatically based on when the appointment has been scheduled for and your Reminder Schedule. You can change when reminders get sent on the Reminder Settings page. You can set them to go out any number of days or hours ahead of time.