Avignon is busiest and most exciting in July, when the Festival d'Avignon fills every courtyard, church, and street with theatre. The festival runs for three weeks and draws hundreds of companies; booking accommodation months ahead is essential. Prices spike and the old town is packed, but the atmosphere is extraordinary.
Spring (April–June) and early autumn (September–October) are the best times for general sightseeing — warm enough to enjoy the outdoor squares, cool enough to walk the walls without suffering. The markets are at their best in spring and autumn when Provençal produce is at its peak.
August outside of the festival can be oppressively hot and the main monuments are crowded. December and January are quiet and cold; most of the outdoor life disappears but the Christmas market on the Place de l'Horloge is worth a look.