The short answer
The quickest route is the calendar's own Notifications tab: open Calendar Settings, pick the calendar, open Notifications and switch on the Reminder notification by email, SMS, WhatsApp or in-app, setting how long before the appointment it goes. For more control, build a workflow with the Customer Booked Appointment trigger and a Wait step timed before the appointment.
Option 1: calendar reminder notifications
Go to Calendar, Calendar Settings, select the calendar and click the Notifications tab. Open the Reminder notification and use its Email, SMS, WhatsApp or In-App tab. Choose who receives it (the contact, the assigned user, or extra numbers and addresses), pick or edit a template, and set how far ahead of the appointment it sends. You can add several timings, for example one day before and one hour before. Send a test, then save. SMS numbers need a country code and each text is billed per segment. WhatsApp reminders need a connected, verified WhatsApp Business account and an approved template.
Option 2: a reminder workflow
Under Automation, create a workflow with the Customer Booked Appointment trigger, using the In Calendar filter to limit it to one calendar. Add a Wait action set to An upcoming appointment or booking, choose Before and enter the lead time, then add a Send SMS or Send Email action. Also decide what happens if that time has already passed, as with a same-afternoon booking: continue, jump to a step or exit. The trigger does not fire for recurring appointments, and guests added after the booking are not enrolled. HighLevel's Appointment Confirmation + Reminder recipe is a ready-made starting point.
Avoid double reminders and blank links
If you switch on both calendar reminders and a reminder workflow, customers get both, so pick one method per calendar. Reschedule and cancellation links only fill in when the message is tied to an appointment, so send them from an appointment-based trigger such as Customer Booked Appointment or Appointment Status, and check them on a real booking rather than a test send. HighLevel notes that calendar notification emails can reach contacts on email DND, while marketing emails still respect it. Book an appointment for yourself an hour or two ahead to see the whole sequence arrive.
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