You need to add this script to a schedule on the server. Assuming you host your file from FileMaker Server (if not you can not schedule the script), Click the "Schedules" tab and the calendar symbol on the right of the screen. There is the option to create a schedule. Just follow the step and you will get your schedule.
This script cannot be run from a server schedule unless you change the script to use SMTP. "Email Client" option is not server compatible.
You could, however, set this to run via a script that runs when the file is opened on a client machine. Assuming multiple users opening your file, the script can log each time that it sends out the email and can check the the log to keep from sending more than once per day.
A robot file run from an OS based "task scheduler" could also do this as a client of the database.