Spring (April to May) and autumn (September to October) are the best seasons for Zhoushan. The weather is mild, the sea is calm enough for comfortable island-hopping, and the light on the water has a quality that makes the archipelago particularly beautiful. Summer is hot and humid but the sea temperature rises to comfortable swimming levels, and the beaches on Zhujiajian are at their best through July and August.
Typhoon season runs from June to October and can disrupt ferry services with little warning — if island travel is central to your plans, have flexible tickets and build extra time into your schedule. The worst typhoons in recent history have struck in August and September.
Guanyin's birthday and other major Buddhist festivals bring enormous crowds to Putuoshan Island; the dates shift with the lunar calendar but generally fall in spring and late autumn. The pilgrimage atmosphere is worth experiencing once, but the crowds are intense — plan to arrive early and leave before noon if visiting on a festival day.