First, you'll need to be able to perform this script at least once a day. If you host this file from FileMaker Server, a Server Schedule can perform this script and use an SMTP email server to send out the reminder emails. If you don't host from server, you can set up a script to do this that runs every time you open the file on a specific computer or use a "robot file" opened from an OS based scheduler to perform this script on a daily basis.
The script would need to be able to perform a find for all such events that are:
a) coming due
b) have not already been used to send out a reminder
a) is a matter of setting up critieria to find records with a certain date or range of dates. b) is a matter of "marking" the record by setting a value in a field and using find criteria that then omits records that have been "marked" in this fashion to keep from sending out unnecessary repeat reminders.