It depends very much what time of year and what part of Italy you wish to go. If you want to go skiing in the mountainous North, January through March is your best bet. Meanwhile, from Tus-cany further to the south, it gets very hot in the summer, with temperatures in Rome, for example, reaching 40 degrees (100+ F) on a daily basis in mid-summer. A general rule of thumb is that June through September is the best time to check out Italy’s many beautiful beaches. Be sure to check current Italy travel advisories before you go.
For city tourism, April, May, and September through November will see temperate climates that make urban exploration much more fun.