This is most likely regulated by the EMail server. Being a WEb Based Email like GMail there wouldnt be a way to do it. Even you ISP you wouldnt be able to. So it seems that this isnt possible since almost all emails are regulated by the email server that send them. Plus even if you could...it would appear new to teh person it was sent to. Since it would arrive today. Not on the day you wanted it to appear...If they really wanted to at that point they could contact the email provider and find out the real date it was sent.
Sorry to say but this sounds like a futile effort.