I have been going over for 10 years or more and usually September, It is quieter (sometimes )then but as everyone says the weather can be hit and miss , but i think you will get more good days than bad days. The worst i have had in France was June 2007 and it never stopped raining for 2 weeks so went again in September and the weather was good
.
Take a pac-a-mac
and you will be ok.
2 consecutive days from 2016 .
top pic La Rochelle bottom Oradour Sur Glane