Late spring (May–June) is arguably the best time: the lake is warming up, the flowers are out, the crowds haven't arrived, and prices are lower than in peak season. The lemon trees are flowering and the olive groves are vivid green.
July and August are the busiest months. Sirmione and the southern lake become very crowded; accommodation prices peak; queues are common at the major sights. The water is warm enough for comfortable swimming. If you visit in summer, stay in the smaller villages on the western or eastern shore rather than the major tourist centres.
September and October are excellent: the summer crowds thin, the grape harvest begins in the wine hills, the olive picking starts in late October, and the lake remains warm enough to swim well into September. Autumn colours on the surrounding mountains can be stunning.
November to March sees many lakeside hotels and restaurants close. The southern lake is quiet and a little melancholy; the northern lake has some ski-touring above Riva. It's a time for the lake's permanent residents, not really for visitors.