Summer (June–August) is the obvious peak season. The lake is warm enough to swim in (water temperatures reach 22°C in July and August), the promenade is busy, and steamer services run on full frequency. Accommodation books up quickly, especially during trade fair weeks — Friedrichshafen hosts Eurobike (the international cycling trade fair) and other Messe events that pack every hotel in the region. Check the Messe schedule before booking.
Late spring (May) and early autumn (September) are the best times to visit if you want good weather without the peak crowds. The lake is calmer, the Alps are often clearer in the lower humidity, and prices drop noticeably. The Bodensee apple orchards are in blossom in May, which is one of the region's most attractive periods.
Winter (November–March) is quiet. Many lakeside restaurants and the steamer services to Switzerland reduce hours or close entirely. The Zeppelin Museum and Dornier Museum are open year-round and are comfortable to visit in cold weather. Occasional fog settles on the lake in winter; when it lifts, the snowcapped Alps behind the Swiss shore are dramatic.
The Seehasenfest in July is Friedrichshafen's main city festival, with fireworks over the lake, music, and a large funfair on the waterfront. It draws big crowds and is worth timing a visit around if you enjoy that kind of thing — or worth avoiding if you don't.