I would say these months; April, May and October are perfect for visiting both cities and countryside in Italy, in particular Venice. But if u are going to ski, the best period is usually from late December to end of March. Afterwards the rates for hotel and ski passes are lower but you may not find really good snow conditions.
Italy is not a hugh country like USA so there are no hugh differences in climate. It falls in the Autumn season from October - November. During this seasons, the scenery is beautiful, the temperatures are pleasant and there are relatively few crowds.
I'm planning to to there on X'mas eve (if poss)
