Scheduled newsletter plugin or functionality?

First, I know how to capture emails and send a “newsletter” based on my new posts to my subscribers. This is NOT my questions.

I want to do a 12 week newsletter (already written and ready to go) that will start and end according to when the person signs up to get the newsletter. So the newsletter would be staggered – not scheduled with each new post. How can I do this? I’m willing to use wordpress plugins if needed, but from what I see, all the newsletter plugins are to send out when you post something new, or you have to do it manually.

Thanks.