First point is that in all major cities of France you can expect traffic jams weekdays at rush hour, from 7 to 9 in the morning, and from 17 to 19 in the evening, except Paris where congestion is omnipresent except at night and mid-summer.
However, the major roads also suffer from congestion now and then, especially during the holidays. Here are the traffic jams days:
Worst traffic jams
13th to 17th July - the first big holiday
Last weekend of July (the busiest of the summer) - 29th to 31st.
The first weekend of August - 5th to 7th.
Really bad traffic jams
Second weekend of July - 8th to 10th.
Fourth weekend of July - 22nd to 24th.
Second weekend of August - 12th to 15th (Assumption weekend).
Third weekend of August - 19th to 22nd (returning holidaymakers).
Last weekend of August - 26th to 28th (returning holidaymakers).
Avoid if possible to hit the road at those times.
Remember France holds the title for the longest traffic jam ever! February 16, 1980, from Lyon to Paris (return from the skiing holidays), due to "many" cars and bad weather, the congestion was 176 kilometers long (109 miles).
Traffic information live and forecast
Enjoy your driving!