Vertical spooning/jigging works year round. It becomes more popular during the colder months because the fish hold deeper and other methods don't/can't reach them. The fish also become more neutral and a vertical presentation keeps the lure in front of them longer. So any time you are ready to start is the time.
It starts picking up the moment the lake starts cooling down. That doesn't mean the bite is hot, it just means they'll start moving more.
I like vert in sept/october personally, because although there are less overall walleye, you can get some serious wiper, catfish, and smallmouth to hit your lines, which always keeps your on your toes.