Hi Ethang, 

Your automation should send as soon as Airtable realizes it is a day prior to the appointment. It is likely your email will be send out at around midnight the day before an appointment. For example if the appointment is on July 28th, the email would trigger at roughly 12am on July 27th.

If this is undesired you can modify your trigger conditions to be of the type "At a Scheduled Time" and set it to trigger at a specific time each day. In the second step, you would use the "Find Records" action to search for all appointments that are happening the following day. You can then create a loop to go through all the found records and send individual reminder emails to all recipients.

Your automation would appear roughly as the image below: 

I hope this helps! If you have any questions or need further clarification, please don't hesitate to ask.

Sincerely, 

Chantal 

Great, recommendation! Going to test this one