Advice Needed: Itinerary for 2 weeks in Italy during December

Why not do "open jaw" flights? Fly into Milan and out of Naples (or vice versa), this saves doubling back.
Then the logical order is Milan -> Verona -> Venice -> Florence -> Rome -> Naples
All easy by High Speed trains.

Two worries.
1) December can be wet.
2) You are travelling over Christmas and the new year. Do check what will be open over these periods. You are travelling on the 25th December.

Day trip from Florence: Pisa, Sienna, Tuscany

But you've only got 4 nights in Florence, you only have time for 2 day trips.
Where do you want to see in Tuscany? Tuscany is a Province, capital Florence. If you want to visit the countryside you need a car. But where? and you have no time in your plan for that.

Day trip from Milan: Lake Como, Turin

Again you only have 2 nights in Milan, so only one day trip.

Train bookings - I take it you are familiar with the Italian Railways website: http://www.trenitalia.com/tcom-en
Timetables change second Sunday in December. Times and bookings past then will not be available until about November. Once they appear, you need to book.
I have no idea what happens to trains over Christmas. Do they get very full?

If you are stuck with having to fly in/out of Milan, I would add a day to Venice and one to Rome and skip going to Naples. This would give you more time for your side trips from Florence and Rome.

Weather likely to make visiting Lake Como or Cinque Terre far from ideal and Amalfi is even farther than Naples from where the rest of your trip is.

I think you're trying to do way too much. For instance your day trip from Florence, if you just went from Pisa to Siena, you'd be spending more time getting from one place to the other that you'd hardly have any time once you got there. I suggest you reduce the number of day trips and only try to make one destination instead of several.

You can't see it all in one trip. You could spend all 16 days in just Rome or Florence and barely scratch the surface.

The good news is that December is a great time to go. The towns will have their Christmas decorations up and you should be able to find some Christmas fairs. It won't be as crowded as it is in the high season. When we were there in December, the temperature was in the 50's to low '60's during the day with a light frost a couple of nights. It only drizzled once, we had brilliant sunshine most days.

As others have said, skip Naples. One full day there is not enough time to see both Pompeii and the Amalfi Coast and you need more time in Rome and Venice.

A more realistic itinerary -- especially given holiday closures (no sights open on the afternoon of the 24th nor all day the 25th) is below. The dates represent the nights you should spend in each city

Dec 16 -- Verona

Dec 17-18-19 -- Venice

Dec 20-21-22-23-24 -- Roma (Don't get too wrapped up on Day trips. There's enough to see in Roma. Ostia Antica would be nice in place of Pompeii.)

Dec 25-26-27-28-29 -- Florence (Day trips possible, but allow two full days for Florence itself. Siena, surely, Pisa, possibly, but "Tuscany" is too broad. You are in Tuscany in Florence, Siena, and Pisa. Christmas Day might as well be used to change locations since sights are not open, for the most part. You will want your hotel to help you make a reservation for lunch and dinner.)

Dec 30-31 -- Milan (Again skip the day trip idea. Lake Como is too far off-season in December. Turin? Maybe if you cannot find enough to keep you interested in Milan itself. Turin is lovely.)

Note that the 16th, 17th, 20th, 25th, and 30th are travel days. You will "lose" 1/2 to 3.4 day in changing location so do not plan on doing a lot of sightseeing. Departure from the prior location including packing, arrival, checking in, getting settled and getting oriented to the new city eat up a lot of time. And daylight hours are short short short. Luckily there are some museums with night hours and we always found it very nice to go at about 18:00 to a museum then have our dinner after 20:00.

Hi Chris, Joe, DougMac and Laurel

Thanks very much for your advice and appreciated.
Based on some of your info, I do some adjusment and re-plan for my itinerary:
My airticket will flight in and out from Milan. How about this route and plans? 
16 Dec: Arrival – Milan -> 16-18 Dec: Venice (stay 3 nights), -> 19-24 Dec: Rome (stay 6 nights), -> 25-29 Dec: Florence (stay 5 night), -> 30-31 Dec: Milan (stay 2 nights), 01 Jan Depart from Milan- end.

Day trip from Rome: Orvieto, Pompeii 
Day trip from Florence: Pisa, Sienna
Day trip from Milan: Turin

Best Regards,
Choy Yuh

I think dropping Verona is also a good idea (regarding your latest itinerary). I was actually going to suggest that.... Well done!

My airticket will flight in and out from Milan. How about this route and plans? 
16 Dec: Arrival – Milan -> 16-18 Dec: Venice (stay 3 nights), -> 19-24 Dec: Rome (stay 6 nights), -> 25-29 Dec: Florence (stay 5 night), -> 30-31 Dec: Milan (stay 2 nights), 01 Jan Depart from Milan- end.

Day trip from Rome: Orvieto, Pompeii 
Day trip from Florence: Pisa, Sienna
Day trip from Milan: Turin

Choy, looking much better! I'm excited for you! I think you'll have a terrific time.