We use Yahoo Groups for our condo messages, so I checked there to see if it had something that could do the job. Yahoo Groups does offer a "monthly periodic e-mail feature", but it is fatally flawed in two ways. First, it doesn't appear to work. Second, it doesn't let you specify when the message will be sent.
Fearing that I might have to write something up myself to solve this problem, I first started searching around for a solution. I was pleasantly surprised to find Letter Me Later. It's great! Perfect solution, very configurable, yet still easy to use. Check it out if you ever find yourself needing an automated e-mail scheduling service!