I will be in Paris for 10 weeks between late July and early October. I'm thinking of taking the train, mid-week, for a 3-night stay in Aix en Provence. Are there reasons to choose either August or September for this jaunt?
I'd play the weather odds and push the southern destinations as late as possible, leaving the earlier dates for side-trips to places like Normandy and Brittany.
No and you know nothing stays the same.
But you never know whats happening at a place when you are planning to go somewhere unless you do research in 2018.
August is very hot and it is vacation time in France, so some businesses and many restaurants could be closed. I’d opt for September.
In my opinion, September would be better unless you happen to pick the week that students are returning to the local universities to start their fall term. They can be quite loud late into the evening, clogging up restaurants and bars, and making the town, again in my opinion, unpleasant to visit. You might do a little research so you don't stumble into that unaware.