How to send an automation on a specific date and to specific customers without a form or calendar?

My site sells physical bartender trainings (to be done on site) but I have to use wix stores to be able to match my needs. I would like to be able to send an automatic email three days before the beginning of a training. The problem is that to allow customers to choose their training date I set the options of an item in date (choice of a date instead of a color). But Wix Stores doesn’t understand it as a date so I can’t set an automation on it. I wanted to know if we could code an automation to solve this situation?
Thanks in advance,
Yours sincerely,