Aix is at its best in late spring and early autumn. May, June, and September give you long warm days, manageable crowds, and the surrounding landscape at its greenest. The Festival d'Aix-en-Provence runs through July, which is the city's grand season — opera in the archbishop's courtyard, jazz on the squares, every restaurant booked — but expect Provencal heat well over 30C and accommodation prices climbing accordingly.
August is hot, dry, and quieter in the city itself as the locals leave for the coast, but the surrounding hills are tinder-dry and the mistral can roar through for days on end. Winter is mild and the markets keep going year-round; January and February see clear, cold air and snow on the summit of Mont Sainte-Victoire. The almond blossom around mid-February is one of the prettier sights in the Provencal countryside.