Late spring (May–June) is the best time to visit Windermere: the daffodils Wordsworth wrote about are out in April, the trees come into full leaf, and the days are long. Crowds are manageable until the school holidays begin in late July.
Summer (July–August) is the busy season — accommodation prices peak, Bowness is overrun on weekends, and the lake roads jam up. Weather is genuinely warm enough for swimming (with care — the lake stays cold below the surface). Book ahead for everything.
Autumn (September–October) is the second great window: lower prices, fewer people, and the fellside woods turning gold and copper. The reliably bad British weather gets worse from mid-October onwards but a clear October day on Windermere is unbeatable.
Winter is wet, dark, and atmospheric. Many smaller attractions close or reduce hours. Snow on the fells with a clear blue lake makes for spectacular photographs, but plan around the weather. The Windermere Marathon runs in late May; Windermere Lights illuminates Bowness in November.