Needless to say that the Carnivale in Venice is one of the biggest events on the Italian calendar. The costumes and colours throughout the event are not done in halves as people of all ages don themselves in anything from pirates to a more traditional medieval masked Venetian ensemble. This year the event kicks off on the 26th of January and runs until the 12th of February - you won't be disappointed.
Other than Carnivale there isn't a lot happening early in the year. There is, however, "The Market Of Miracles" which is the only antique and second hand market in Venice organized by non profesionals. This happens in March over various dates.