google - weather Milan or weather Venice etc you should get lots of site options

http://www.weatherbase.com/weather/city.php3?c=IT&refer=

long term averages

Thanks, the average are easy to find, I'm more interested in if there were any 80 degree days in the area at that time last year.

We are struggling whether we should be dressing for fall or late summer.

Layers, as you could be experiencing both late summer and fall, too hard to predict.

This type of thing you cannot be decide with certainty. Even within next few days, the forecast vs. reality are quite off. What happened last year has little bearing on what would happen this year. I think you need to reformulate your strategy.

Here is one resource for upcoming month:

http://www.weather.com/weather/monthly/ITXX0042?month=1

Based on historical weather data, assemble wardrobes that can SPAN the variations. Within 10 days of your departure, you start to be able to see more specific weather forecasts to narrow down the amount of variations you need to deal with. These are still quite ways off. Many folks dreams/want/think it is possible to put all eggs in one basket by taking exactly the correct type of clothes. This would be like a lottery. Overtime, experienced travelers end up assembling wardrobes that can span variety of weather without taking up a lot of luggage space.

The best resource for historical weather data is Weather Underground!

Their Travel Planner is a terrific tool:
http://www.wunderground.com/travelplanner/index.asp

Or you can just look at historical data for a specific location and single date this way:
http://www.wunderground.com/history/

Looking just at last year would be useless. this year could be completely different. You may want to look not at average but highest highs and lowest lows - but that would force you to bring two separate wardrobes.

IMHO you will need some summer weight things - but also more layers since it will likely be cooler at night. I feel fairly sure it won;t be 100 degrees - nor will it snow - but you can get a lot in between.

Looking just at last year would be useless. ... You may want to look not at average but highest highs and lowest lows

That's why the Weather Underground travel planner is so great! It provides statistics and charts, plus comments like "Of 225 days between 1996 and 2012, Partly Cloudy was the most frequent condition. Additionally, 91 days were recorded with precipitation." and "We are confident that the weather will be Warm. The wind speed averages indicate that the temperature will feel a bit cooler than its recorded value."

(To get these examples, I plugged in planning a trip to Paris for a random 2-week stretch in July 2012 - I know the OP is going to Italy...)

Even if the temps are not as high as 80F, it often feels warmer because of higher humidity. Plus, you'll be moving around, walking, climbing stairs, etc. The answer is not a cyber crystal ball but as others have suggested: clothing layers that can be added or subtracted for comfort. FYI, you won't find building interiors to be overly air conditioned.

I was there last year when you will be this year, and it was hot. We lay by the pool at our airport hotel the last night in Rome. There was no rain the entire three weeks we were in Italy.

The year before I was in Rome the first week in November,and it was in the high 70's F with thunderstorms.

A couple of years before, I was in Venice in mid October, and it was polo shirt weather.

I have been to Tuscany in late October one year (Siena and Val d'Orcia)and had frost.

Another time, we were in Venice in early November, and it was cold and so foggy that we could never see Sn Giorgio Maggiore from San Marco.

See the problem? But I would bet on late summer rather than early fall.

So - what's on your packing list now?
Mine had
Hat for the sun
Small fold up umbrella
Light weight jacket that was rainproof
to be combined with
a sweater
Pants, shorts, shirts, skirts
and walking shoes plus another pair that looked nice in restaurants at night.
Depending on you, a bathing suit, flip flops.

No baseball caps, or Tshirts that say "I'm with Stupid"
How is your trip coming out?

I'll be in southern Italy the same time as you and struggling with the same questions: weather, what to wear, what shoes to bring. So your questions are helpful!

Did you even look at the data I suggested?
More than you could ever ask for. Your temps will vary widely for your various locations.