Venice's tourist season is very nearly an all-year affair. Peak season is from April to October , when hotel rooms are virtually impossible to come by at short notice; if possible give the central part of this period a miss, and at all costs don't try to stay in July and August, when the crowds are at their fullest, the climate becomes oppressively hot and clammy, and many of the restaurants close down anyway. The other two popular spells are the Carnevale (leading up to Lent) and the weeks on each side of Christmas ; again, hotels tend to be heavily booked, but at least the authentic life of the city isn't submerged during these festive periods, as it is by the summer inundation.
For the ideal combination of comparative peace and pleasant climate, the two or three weeks immediately preceding Easter is perhaps the best time of year. The days should be mostly mild - though the weather can be capricious - and finding accommodation won't present insuperable problems. Climatically the months at the end of the high season are somewhat less reliable: some November days are so clear that the Dolomites seem to start on the edge of the mainland, while others bring fogs that make it difficult to see from one bank of the Canal Grande to the other. However, the desertion of the streets in winter is magical, and the sight of the Piazza under floodwater is unforgettable. This acqua alta , as Venice's seasonal flooding is called, is an increasingly common occurrence between October and March, and you should anticipate a few inconvenient days in the course of a two-week visit in winter. Duck-boards enable people to move dry-footed around the busiest parts of the city, but some low-lying areas - such as around Campo San Polo - become impassable to anyone without gumboots, and on certain freakish days the water rises so high that boats can be rowed onto the Piazza.
If you want to see the city at its quietest, January is the month to go - take plenty of warm clothes, though, as the winds of the Adriatic can be savage, and you should be prepared for some rain.