If the file is being hosted, you could have an "on timer script" running the email notification script on the server and set it to run daily or whatever time period you want.
If you are using FileMaker Server, a server schedule can perform such a script on a daily basis.
A FileMaker robot file can be run from a system task scheduler that does the same thing once a day.
An on open setting can also be made to work if you set up the script to log each time it runs in a separate table. The script can then check this table and not do anything if the script has already run once today.
if u have filemaker server, u can schedule a server script that runs automaticaly (every day at 4am for example) and check for the cretaria u wan, in that case mails will be sent automaticaly to users,
The only option to do this automaticaly is to use filemaker server, (unless some one got another solution that i dont know about )