The best months are April through October. Mainau island is at its most spectacular in spring — tulip season runs through April and May, and the rhododendrons follow. Summer brings the full ferry schedule, lake swimming from the shore beaches, and long evenings on waterfront terraces. The water temperature in the Bodensee is genuinely warm by July and August, and swimming from the public beaches east of the old town is a local ritual.
Summer is the busiest period. Mainau in particular draws large crowds from late June through August. Arriving early in the day helps, or visiting on weekdays rather than weekends.
September and October are good if crowds are a concern: the weather holds, the lake is still swimmable into September, and the tourist volume drops sharply after school holidays end. The vineyards on the surrounding hills are harvesting in October, which adds another reason to visit.
Winter is quiet. Most lake cruise operators reduce to a skeleton schedule and some stop entirely. The old town is pleasant to walk in the cold, and the Christmas market fills the streets of the Niederburg with the usual German combination of mulled wine, candles, and carved wooden ornaments. The market runs through December and is well-regarded but not exceptional by regional standards.