Last year - without planning it - I happened to be in Venice during the annual Carnivale. I was staying at the Marriott Dei Dogi Hotel in Cannaregio, a fair distance from San Marco Piazza, where most of the activities are focused.
This year's Carnivale begins February 16, and if you've never been there during this festival - akin to the Madis Gras in New Orleans, U.S. - it's quite fascinating, with all the theatrics, costumes, plays, crowds, etc.
If you're interested in this year's event, check it out here:
Suggestion: Unless you prefer staying in hotels near San Marco, you might want to consider the Marriott Dei Dogi, as mentioned, or the Marriott AC, near the train station and bus park, where rates might be slightly less during this two-week event.