Spring: Late April through early June is a good time to visit Venice: the weather is mild, but the volume of tourists is larger than it is in summer.
Summer: Summers are warm and humid. The advantages are fewer tourists (but their numbers are still substantial), it almost never rains, and the beaches of the Lido are just a boat ride away.
Fall: Autumn, like spring, is a good time for visiting Venice. It's usually pleasant and sunny well into October, and it doesn’t really begin to get cold until mid-November. Acqua alta is most frequent, and severe, in later November.
Winter: Venetian winters are relatively mild, with frequent rainy spells, but also many more sunny days than there are in Northern Europe or much of North America. There are substantial crowds during Carnevale, and prices for hotels and even in some restaurants skyrocket.