This year the Jazz festival in Montauban is running 27th June – 7th August. Full details can be found on the official site here.
If you are travelling to the festival then train is an excellent way to get there. Montauban is at the crossroads of the rail links Bordeaux-Nice and Paris-Toulouse-Spain. Worth checking out Rail Europe for details.
As always check Holiday Currency Exchange for the best Euro rates before you travel.