Temperatures in Venice are much more moderate than in Florence or Rome. But personal experience tells me that when it's moderately hot in Venice, you swelter. It's the humidity.
Unlike the other cities of Italy's "Big Three," Venice's rainfall is pretty well distributed throughout the year, totally just over 32 inches on average. You'll find thunderstorms, particularly in Spring and Autumn. The biggest holiday periods in Venice feature its worst weather: Christmas (wet and cold), Carnevale (foggy and cold), and Venice Film Festival time in August (hot and humid).
The best times of the year to visit Venice are from May through early June, then September and October.
Here are links to more information on planning your vacation to Venice, Italy.
Venice, Italy Travel Essentials
Venice Bed & Breakfast - VeniceBy.Com
Venice Airport Hotels (Marco Polo Airport VCE)