There are presently 2 challenges: Panorama which will end at midnight tonight and the other one is "Models" and it will end next sunday night at midnight. I don't know where you have seen the "Changing seasons" one, but I suspect it might be from last year or the year before.
The only thing that comes to mind is that maybe you are looking in the "weekly challenge" forum but you are seeing the last ones first. In your settings, you can list the threads so that the newest ones come on top. This way, when you look at a thread, you'll see the latest entries first.
If you go to the weekly challenge forum, you should see all the entries that came in so far. There were quite a few.
Welcome to the forum.