When you start a new topic, there is an option in a checkbox if you scroll a bit down below the text box to make it an "event" post. There you can set a date for the event. When you do that the event appears on the calendar that you can access at the far right of the links bar on top.

So by checking that box it only means that it is a topic with a date associated with it, for example a post taking about an upcoming party or meeting date or something. I haven't seen a post like this yet but I assume that is what you are talking about. Can you let me know where you've seen it (if you can find it again ) I'd like to check it out.