Depending on the server OS you are running there are different ways of doing this. I only know of how you'd do it using a *nix OS.
Create a web page that generates and sends the emails, watch out for timeout issues if you are sending alot.
Create a cron job that runs at the specified time and runs lynx calling your web page.
Or
Create a php command line script (need the shell version of PHP installed and not the Apache module) of the above web page.
Create a cron job that runs at the specified time and runs this script.