Using Automator To Send Repeat Prescription Email

I have a repeat prescription which is triggered by a request email from me, to my local health authority every 28 days. At the moment, I automate this email with a script file stored on my iCloud which is opened by an alert function from the calendar on my Mac.

The problem I have is, if my Mac isn't running when the alert time occurs, the file doesn't open and the email isn't sent.

Is it possible to create a script that would fire this document after 28 days at a selected time, or if that is missed (because I'm not at my computer), when ever the Mac is next booted?

Thank you!