I wouldn't directly invite them to post, or even that they didn't post for x-days. Comes of as begging and/or annoyance. You need to do it in a way that the user doesn't get the impression that you want them to do something, and somehow the user must feel that there is a reason to post, because there is an advantage over not posting.
The mod could still count the days, and show a message after x-days without posts. But instead of pointing the finger at the user, why not inform them of the benefits of posting?