Arles is best in spring (April–June) and early autumn (September–October). The light is softer than high summer, the crowds are thinner, and temperatures are comfortable for walking between the Roman monuments.
Summer is hot and can be crowded, but July brings the Rencontres d'Arles photography festival — one of the world's best — and the arena hosts events through the summer season. The Feria Pascale (Easter) and Feria du Riz (first weekend of September) bring Provencal bullfighting back to the Roman arena, which is an extraordinary experience whether or not you have any interest in bullfighting.
Winter is quiet, some restaurants close, but the city is beautiful and uncrowded. The Saturday market runs year-round.