Any question answered...

Quick google says:

...The worst weekend in France throughout the year is one week before the late summer Bank Holiday (late August - U.K.). The crossover of the "juilletistes and aoutiens" (July and August holidaymakers) in late July sees millions of people moving across the country over the weekend with travellers either starting their holiday or returning home...

http://www.day-tripper.net/drive-in-france.html

...and: http://about-france.com/week-planner-2012.htm