Which months to travel to Italy

Sept will be more crowds, hot but less chance of rain/stormy weather
Nov would be much less crowds, not hot but good chance of rain/stormy weather
October would be in the middle

Depends I guess how far so but think October would be ideal for me for Sicily, I would prefer Sept for Rome/Naples/Amalfi area.

We often look to go to Europe the last of March--right before the more expensive Spring airfares come in. And the winter weather's usually just broken.
The Fall's interesting in Italy as the agriturismo's are in the midst of their harvest.
I once was in Venice in February, and the weather was freezing and windy. They had elevated wooden walkways all over St. Marks Square. It really wasn't a very pleasant day trip.

October hands down. Early to mid September doesn't feel that much different than August, which is sweltering hot.

October for sure. We have done two trips (Oct. 2015 and Oct. 2016) and both times the weather was wonderful, quite warm enough for walking and sightseeing (20 C), clear skies, no rain, except for the very last day (Nov. 5) in Venice. Perfect!

Our preference is always mid-Sep to end of Oct. Daylight is too short in Nov and even late Oct.

October, or very late September into October.

This might be of some help:
http://adventureswithsarah.net/best-time-to-go-to-europe/

(Sarah is a Rick Steves tour guide who specializes in Italy).

A year ago early March we were in Milan then down to Florence and Salerno--it was pretty chilly most of the time, 50's during the day, 35-40 at night with a wind that residents say was uncharacteristic. Next year we will be in Rome down to Sorrento about the same timeframe. Supposed to be fairly temperate along the coast with highs in the 50's, lows in the 40's.

I have never traveled to Italy in the summer, once in October and that was enough. Enough tourists & queues to make it unpleasant at times. We travel off-season and the ease of getting around, the lack of oppressive heat, and chiefly the lack of crowds more than offsets any inclement weather that might be encountered.

All things considered, however, the optimum times to travel to Italy in my eyes would be April 1 in the south, mid-to-late April in the north.

Caveat - I am averse to heat and humidity. I've traveled several times in February and loved it. My one time in mid-May was pleasantly warm from Bologna to Orvieto and hordes of tourists only in Florence. My only time in fall was mid October - Venice was okay, in Florence I was grateful for AC in my room every night. That's as far south as I've been except in February.

When you say "southern Italy" are you talking about the Amalfi Coast, Puglia, Bari, etc. Or do you mean around Rome and Naples?

We book our trip according to fares and weather. Weather wise we like fall and spring. We have always had good luck for fares in April and May. You just have to look every day because they can rise and lower quickly. Kayak on a Tuesday or Wednesday usually works but we look at he fares for quite a long time before we 'pull the trigger' and get the fare we like. A month ago fares were very high to Rome, now they have dropped for the spring. Also, always cheaper to fly in to Milan. So, spring time is our time for italy and Europe in general but before the crowds of the summer

We've been in May in 2013 and 2015, weather was pleasant both times.
Regards

As Chani says, it does depend on what you mean by "Southern Italy." It also depends on individual taste. For instance, when I went to Rome in September, I loved eating dinner outdoors every night (all the restaurants have outdoor tables, so the whole center of the city is like one big block party). One friend agreed with me that this was a big part of the experience. But another friend didn't; he hated the heat, and didn't feel that eating dinners outdoors made up for it. He preferred going to Rome in April, when lunches were outdoors but dinners indoors.

I was in Sicily in late April and early May of 2014, and had fantastic weather. I was in Rome and Naples in September 1994, and while it was a bit hot for my taste (particularly in Naples), I did fine.

If you can go any month, check not only weather averages, but airfares. September is usually still high season for airfares to Italy.

It depends on what you want to do. If you like to spend time at the beach, November is not good.
Check weather averages at the locations you want to visit and decide.

No doubt in my mind. October.

Like many others here, my favorite times in Italy would be late Sept through October, or late April through May. Of course it depends quite a bit on which regions of Italy you plan to visit, and what activities you want to include (beaches, skiing, inside activities like museums, etc.)