If possible, I second+++ flying out of Venice. The airport is small and super easy, compared with trekking out to the airport in Rome or backtracking to Milan. I would add a day to Rome, as it is huge and so much to see. I personally love staying in the Jewish quarter. It is well located and not very crowded. I know you are trying to squeeze as much as possible into your first trip to Italy, I been there, done that. San Gimingiano is a favorite little village and a fun place to stay and break up all the big cities. I stayed at the Lyon Blanco on the town square. Personally, I would skip Pisa...it's pretty one dimensional. The tower leans...okie dokie. Sienna is worth the time. Florence, is a two day city. IMO. Purchase a combo half day tour package and see the David and the Ufizi. You skip he lines and get some good info. Venice is fun, but be prepared for serious commercialism. Head in from the Grand Canal for some relief. Have a wonderful trip.
Tina

meant to add that the weather at Christmas is often quite beautiful in le Cinque Terre & great for hiking/sightseeing, but group tours out of Florence often will cancel if there are not enough people or weather is bad. If you want to go, read up ahead of time about who is doing tours or how to get there on your own (rains will be crowded) but don't actually book anything until you are in Italy and know that it will be sunny.

all right. thanks all. I think I'll have to skip Cinque terre trip, will roam in Florence for 1 day.
Flying in Milan on 23rd, staying on 24th, out to venice on 25th (stay 1 night), Venice to Florence (3 nights), Florence to Rome (3 nights), flying out of Rome.
now the bigger question. booking train tickets. checked out the italiarail website for train tickets. It says, no tickets match your criteria. OOPS! is is too early, or no?