Weather is a major part of vacation planning in Italy, especially if you are lucky enough to be able to vacation in the off season.
The cities in the map below are the major tourism areas of Italy. For each location, you can click on the city or region name and get taken to charts telling you the average high and low temperatures for each month, and the average rainfall throughout the year. You will also get "when to go" information.
The tourist hot spots on the map are crowded in summer. Italy is a popular tourist destination. I like to travel in the fall. Late September to Early November is the best time for me--the best light for photography and the best food. Truffles fairs start in November for the superior winter white truffle, and the harvest and wine fairs are also going great guns in the fall.
Flower lovers (especially if you like Europe's red poppies) will be better off chosing a spring vacation. The middle of April through June is fine for the north of Italy. By June, the south is starting to get hot, if that's your thing.
One of the secondary considerations of tourists visiting Italy in the off season is the number of hours of sunlight there will be. The point of tourism is, after all, to see things. Euroweather has a nice calculator to determine just how many hours of sunlight you'll see in a particular city on a particular day:
Of course, eating dinner underground a cantina by a roaring fire in winter is, to me, equally compelling as eating outside as the sun slowly sets over the Adriatic.
You can get a wider variety of city weather and climates on Travel Weather: Italy Weather and Climate Map. Travel Weather has climate information on all of western Europe.
Wanderer's Eye Photography
Travel Photography from Italy and Elsewhere
Europe Travel
C'mon, take a trip to Europe
Italy Travel
Travel Information on Italy
La Cucina Povera
Food the Traditional Way