Nîmes is hot in midsummer — July and August routinely touch 35°C — and the city largely empties on weekends as locals head for the coast. April to June and September to early October offer the best combination of weather and atmosphere: warm sun, manageable heat, terraces open, monuments uncrowded.
Time a visit around one of the city's big events if you can. The Feria de Nîmes at Pentecost in late May or early June and the harvest Feria in September take the city over. The summer concert season in the Arena runs from May to August, with major international acts and opera. The Jeudis de Nîmes — Thursday-night markets and street performance — run from June to September.
Winter is quiet, mild, and an acceptable time to visit if you want the monuments to yourself. The Christmas market in Place du Marché is small but pleasant.