You can post at any time to midnight on the last day of the month (midnight in your own timezone as I understand it). Most people post nearish to the deadline but it depends if you are done I suppose!
Usual form is one final render (which can be a compilation of shots but not with effects and things) in the final render thread and then most people tend to post a few other last shots in their challenge thread.
There is not the same size limit on finals as for progress renders. Have a look at a few previous challenges to see what was being done - the final render thread is a fairly new thing as it was hard to tell which shot people wanted to use and this way it's clearer.
Hope that helps and best of luck to you too 8).