I am speaking from my own experience right now! During the summer it is extremely hard to breathe! You will sweat and sweat, continuously! I had some really bad pictures because of that. Anyway, I also got the chance to feel their "autumn". October was a really cold month! I felt the cold to the bones! I imagine that April and May are the best month to visit based on my experience!
I read somewhere that Aril to June and September to October is the best time for travelling in Italy. At that time the weather was good not so hot and not so cool. At this time there are so many of people are coming to visit and enjoy a lot.
The best months for traveling in Italy are from April to June and mid-September to October—temperatures are usually comfortable, rural colors are richer, and the crowds aren't too intense. From July through early September the country's holiday spots teem with visitors.