Try going into Options for the message and selecting the "Do not deliver
before" option.  The message should go into your Outbox and sit there
until the time you specified.  Two caveats, though:  First, when the
message is delivered, it will still show as having been sent when you
first sent it to the Outbox.  So if you write it today and hit send
today, but it doesn't actually go out until Wednesday, it will still
show as having been sent today.  The other thing is that you must have
Outlook running at the time you want the message to go out, or Outlook
won't be able to send it -- it would go out the next time you opened
Outlook.