In Venice, the busy season for tourists is long - extending from Easter until early October, though some would argue that Venice never really has a low season anymore for tourism.

Weather and climate

Venice has a relatively temperate climate, with temperatures rarely descending below freezing during the day, or rising much above 35°C.
July and August are the warmest months with average maximum daily temperatures of  27° Celsius, and the tourists swarming around Piazza San Marco. Average daily minimum temperatures are 19°C in July and August. Humidity can be high and the combination of high temperatures and high humidity can be oppressive, particularly in very crowded streets.
April, May, June and September and October are all much pleasanter months for weather with average daily maximum temperatures ranging from 17°C in April to 24°C in September and 25°C in June.

Weather in September is nice; average daily highs around 75°F/24°C, and evenings around 65°F/18°C.

Winters are relatively cool and often rainy. Snow is infrequent, but possible, usually December through March. During the Winter months fog is also possible, particularly in the morning.

Here is a link to the six day weather forecast information for the province of  Venice, Veneto, Italy.

Flooding (acqua alta)

Exceptionally high water level (acqua alta) is possible at any time of the year, but is most likely in the winter months, and is signaled by the sounding of civil defense sirens when the level exceeds +110cm above sealevel. To better cope with the numerous flooded pedestrian streets, the city's government has developed a plan to enable people to move dryly about many areas of the city, by setting up temporary raised platforms (passarelle) in the middle of major pedestrian walkways which are most subject to flooding, and linking them to many of the public transportation boat stops. These platforms make it possible to walk without getting wet; however the walkways are narrow, just allowing single file in each direcction, small crowds can form at the ends of the walkways, and they do not reach all possible destinations within the city. The flooding of acqua alta is generally linked to the lunar tidal level, in two roughly twelve hour cycles per day (6hrs rising, 6hrs falling), meaning that even when flooding does occur, it rarely slows or blocks pedestrian traffic for more than a few hours before the waters recede again below sidewalk level. Most areas of the lagoon city are over +100cm above sea level, though some lower areas, such as Saint Mark's Square, start flooding at levels of +80cm above sealevel. The three-day forecast for the water levels is always available on the official Venice Municiple site (Maree - Comune di Venezia - in Italian).

When to go

The best time to go depends on your interests. There are important events throughout the year, from Carnevale in the 10 days ending on Shrove Tuesday, the patron saint's Festa di San Marco on 25 April, the Voga Longa (a major rowing marathon) on the Sunday after the Ascension, the Art Biennal in odd years from June through November, the Festa del Redentore the 3rd weekend in July, the Venice Film festival in late August, the Regata Storica on the 1st Sunday in September, the Venice marathon on the last Sunday in October. There is a large pick of art exhibitions throughout the year, which change periodically, besides numerous world class permanent museum collections open year round.

In the low season periods between the Epiphany, January 6, and the beginning of Carnival (February-March), and around the August 15th Ferragosto holiday, a number of restaurants and hotels will often close for short periods of vacation and maintenance, though most of the museums and other tourist destinations remain open.