As campers, we prefer June and July, because at that time of the year we get the maximum amount of daylight. The sun rises at about 6am and sets about 10pm. However at that time of year it can get very hot, with many days over 35ºC.
Our solution to that problem is to get up with the sun, have a quick bite to eat, pack up and be on our way before 7am. By the time the day heats up, we are approaching our destination, even after a stop for a second breakfast along the way. An early start also means the opportunity for photographing in a beautiful even light.
We usually come back from dining in the local restaurant as the light is just beginning to fade, so we never have to grovel about in the dark getting ready for bed.
For non-campers with the benefit of electric light, there is a wider choice of suitable months, say from April to October. Outside of these times, a lot of amenities could be closed. The French themselves traditionally take their holidays in August.