The best-known Carnevale in Italy is of course in Venice. Having lived in the area many, many years, I consider it a tourist trap to be avoided. Venice is already crowded as it is, and at Carnevale it becomes suffocating. Although they do indeed have some marvelous costumes and masks (amidst a sea of gawking, plainclothes spectators). Another famous Carnevale is held at the Tuscan seaside resort of Viareggio, with a big parade with numerous floats.
This year Carnevale will be from January 25-February 5th.