Sending Automated SMS Messages

Easiest way is to use an SMS gateway who provide an API. Check out txtlocal

If you use a provider such as txtlocal you have 2 options - you can either build the scheduling into your system, or you could have a batch process which sends the sms info and the time that you want it to be sent using their API.

I've used Clickatell in the past.

They have a RESTfull API, which means sending as SMS is as easy as constructing a URL with the message and recipient's phone number.

It's not free, obviously, but it's pretty darn cheap.