What is the easiest way to have Apple's Mail send an email at a specific time/date?
The easiest is to simply set the time in iCal and set the alert to email the details.

If that's not exactly what you want, you could have iCal launch an Apple Script to send a specific draft, but there isn't a built-in feature to handle everything from email.

I suppose you could use the scheduled wake event to wake a sleeping mac and have things ready to go, but this seems more likely to backfire and send it earlier if something else wakes up your mac or you open mail and have an internet connection before the appointed time to send arrives.