It's tough to identify the wettest season in Venice. There is more rain in the summer and later in the year—the most rainfall traditionally come in June, August, and November—but there is more water coming from below the city in the winter. Temperature-wise, Venice can climb to 25°C (77°F) in the summer and temperatures hover just above freezing in the winter. You'll need a jacket in the spring and fall.
The acqua alta, or periodic flooding, can begin as early as late September or October, but usually takes place November to March (there is no way to predict them in advance). Venice’s lagoon rises until it engulfs the city, leaving up to 1.5 to 1.8m (5–6 ft.) of water in the lowest-lying streets. Piazza San Marco, as the lowest point in the city, goes first. As many as 50 floods a year have been recorded since they first started keeping track in the late 1700s. Remember, though, the waters usually recede after just a few hours—–there is no need to get wet and the city doesn’t shut down. Walkways are set up along the main routes, but if you intend to wander around, do as the locals do and buy rubber wading boots, available from most stores from 20€ (souvenir shops and stands in Piazza San Marco also sell disposable knee-high plastic waterproof slippers for about 10€, good for a couple of days). A complex system of hydraulic gates—the Modulo Sperimentale Elettromeccanico or just “MOSE”—is being built out in the lagoon to cut off the highest of these tides (controversial because of its environmental impact and the seemingly endless delays that have plagued construction); it is expected to be operational sometime in 2022.
The masks, costumes and galas that have become so synonymous with Venice ring in the season of Lent during the pageantry of Carnevale (www.carnivalofvenice.com) in late February/early March. In July, Redentore, the Feast of the Redeemer, is a giant party on the water with fireworks and hundreds of illuminated boats. Oarsmen show their prowess in the Historic Regatta, a centuries-old race that takes to the water in September. The world-renowned contemporary art exhibit that includes the Venice Film Festival, the Venice Biennale (www.labiennale.org) completely takes over the city in the summer and fall of each odd numbered year while thousands of worshippers parade to the looming Salute church for the Madonna della Salute in November.