I seem to remember that at least PP closes around the third week of September but I could be a bit off on that. Weather can be a bit dicey and I think maintenance crews are more worried about getting the ranger cabins shut down, drained and winterized to avoid freezing and broken pipes. Just a thought.
"Only those who will risk going too far can possibly find out how far one can go.”
A bit dated, but I entered Dawson/French Lake on September 8, 2014 (a Monday). It was the first day of self-permitting that year.
From other years, I seem to remember that the ranger stations in the north were closed in roughly that same window (although the park office in Atikokan stays open). Ones in the south, perhaps a week later. However, I made a trip through Prairie Portage and departed on September 10, 2010 (a Friday). I reasonably sure that the Ranger Station was already closed then, and we self-permitted. I do remember that the Truck Portage (which we used that year) was already working a restricted schedule.
"If you can keep your head when all about you are losing theirs" chances are you missed something. (Inspired by Rudyard Kipling.)
Always a work in progress. Sort of like what hours are they open. The times vary from year to year and within the same year. I think the same is true for when they are closed, may not even have set a date yet.
It changes each year, but you can go to the online Quetico reservation site and it will show you when they last accept reservations. Go here: https://reservations.ontarioparks.com/Quetico/Backcountry?Calendar In 2017 it appears that September 17 is the last day that Prairie Portage is open.