Add a event with a keyword, and write the message in the description
If
Event from search starts
This Trigger fires a set number of minutes before the starting time of an event on your Calendar that contains a specific keyword or phrase in the event’s Title, Description, or Location. Ingredient will be based on the zone set in Google.
Then
Send an SMS
This Action will send an SMS from your Android device to any phone number you specify.
Explore more great ways to automate Android SMS and Google Calendar