You will need a cron job (search these forums for "cron" and you'll get a lot of information about what these are how they work and how to use one to trigger a PHP script at a certain time). Basically there's a program that sits running in the background of the system that regularly looks at the clock, looks at its schedule, and sees if there are any jobs listed there to see if they should be done at that moment.
cron is a Unix beast, but there is a corresponding thingy for Windows systems (I forget what it's called right now).