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 or tap on the city or region name and get taken to charts depicting the average high and low temperatures for each month, and the average rainfall for each month throughout the year. You will also get "when to go" tips and travel information for each Italian city.
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 choosing 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.
The coastal areas of Italy exhibit the typical Mediterranean climate, which means dry (but often humid) summers and mild winters.
Inland you'll find cooler and wetter weather, especially in the off season.
The western side of Italy gets more rain than the eastern coast.
The south of the country, including the "big" islands of Sicily and Sardinia, can get very hot indeed. I've experienced summer temperatures in the 50 degree C range in central Sardinia.
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 next to a roaring fire in winter is, to me, equally compelling as eating outside as the sun slowly sets over the Adriatic.
European Travel Weather has climate information for mech of western Europe.