I've added support to ar_mailer to be able to specify the date on which to send it.  I was trying to use mail.sent_on
to pass the date into the model create call but that has the side effect of adding a Date header to the email.
So instead
you call Email.set_send_on Date.parse(params[:send_on]) just prior to calling deliver on the message.

I added a named scope called ready_to_send that is called by ar_sendmail.

This is a patch against ar_mailer 1.4.0