[Erwin]

You can do this as a template mod (although Admin CP is also not hard at all to do).

Just using templates:

1) Open phpinclude template.

2) At the bottom, at:

$emailmessage = "This weekend is.... blah blah blah";

3) Open email_notify template and add $emailmessage wherever you want it to show.

This is a quick and dirty way to do it without hacking.

Btw, you can now add $emailmessage anywhere in any template, and your message will show too.