Early May or Mid September to tour Italy

We've traveled to Italy in September twice. There are still crowds - mostly noticed it in Florence, but we enjoy traveling in September because students are back in school, the evenings are nice to sit outdoors in the piazzas for dinner, etc.

I packed some cotton-fabric tops along with a few dresses because they were more comfortable in the warm weather ~Rome, etc. and brought a cardigan sweater for evenings along with a light rain coat.

Checking the average monthly temperatures, days of rain, & hours of sunshine, it appears that either would be equally fine.

I Prefer September. Sunny days, cool nights.

I've been to the cities you mention (except Amalfi) in May as well as September. I found the weather to be good at both times. However, in May, there were many school groups traveling – mostly junior high and high school age. I noticed this particularly in Florence. I stay at fairly simple hotels (<100 E/night when possible) and I had a number of sleepless nights, with kids running around the hotel partying. It sounded like a slumber party, and when I called the front desk, they simply transferred me to the chaperone's room – that didn't help much, since we usually didn't share a language. It was an unpleasant enough experience that I've avoided May since then. However, if you stay in more expensive hotels, this probably won't be an issue.

I've been to Rome and Florence in Mid - May and Rome, Florence and Venice in early-September and again in late Sept. Of course it depends on the weather for the year, but I found mid-May 2013 to have more comfortable temps than mid-Sept 2014 and early Oct 2014. I don't know where you are from but you might tolerate heat better than I can.

As stated upthread, I did notice a lot of school groups at the Forum and Colosseum during the mid-May time frame. It didn't bother me and I, frankly, was pretty jealous. Imagine a Field Trip to the Forum! Wow!

I've only been in September and not in May on my past trips. The crowds in Rome and Florence were still there. I like the harvest season for the grapes and visiting the vineyards in september. I'm going back this september but my next trip in 2016 I'm looking at May for more colors in the landscape for photography.

I have been both fall and spring. I prefer spring because you get to see the colors of Italy. Fall is warmer, dustier and hazy for most vistas. It is nice but I prefer the colors. The weather is better in fall (e.g. little or no rain).

Thank you all for your responses. I'm really leaning towards September. I am going to decide in the next two weeks or so. Any other repsonses or thoughts are greatly appreciated.

I'm going to add my vote for September. When I studied abroad in Florence (several years ago, back in 2008), I noticed a distinct difference in crowds post-Easter (as opposed to pre-Easter). As Easter is quite early in 2016 (Google says March 27th), I'd think you'll have more crowds any time after that, especially with (as several others have noted) the school groups on spring break trips. In 2013, my husband and I spent the month of January traveling through Italy, and though we had some cold/rainy days, with the right clothes, we were fine. It's always been my view that you can pack to manage the weather, but there's nothing you can do about managing the crowds :)

September is the busiest month for our tours in Italy and May is second busiest. Either is a great month to travel, but not without crowds or heat.