Weekly Email Reminder Plugin ???

Does anybody have any knowledge of a plugin that would each week send an email reminder to all members of a specific user group to remind them to do something?

I am looking to expand on the Appointments+ plugin where by I can send an email message to all service providers to remind them to send their availability for the next week too me. I want the email to go out each week on a sunday evening. The content of the email need never change it is just a way of reminding the service providers to let me know their availability.

Rather than tinker with the Appointments+ plugin I thought it would probably be easier if there was a separate plugin that achieved this.

Thoughts or suggestions welcome.