Frankly, Provence is charming year-round, but the best time to visit is when traffic is low and the sun is still au Rendez-Vous For the French, August is the peak vacation period, and, therefore many take their vacation during that time. Provence or the Côte d'Azur being high on the list of places where the French enjoy their holidays, they are both flooded with France-bound tourists. July is the second busiest month. If you can, try visiting in July when it is somewhat less crowded and possibly less hot.
During the peak season, July-August, hotels are more crowded and reservations are required.
Between April and May and again until the end of September, weather is usually very pleasant being on the balmy rather than the hot side. Roads, towns and beaches are less crowded, but all hotels, restaurants and attractions are open. Reservations are still required for the hotels. As always, the better hotels tend to be booked a long time in advance, so try to book early. Week-ends can be booked also due to the French week-enders taking a peek at the Provencal sun.
Off-season, mid-September to Easter, there are no crowds, but not everything is open. Many seasonal businesses close during the winter, or at least take their month's vacation during this time, including many hotels. Restaurants can close early.