Flights at this price are available daily starting on October 28 through the end of March 2019, which happens to overlap with Venice's world-famous Carnival. Carnival dates in 2019 begin on Saturday, February 16 and culminate on Tuesday, March 5th.
Searching on the Swiss website, I could easily find multiple dates that fall during the peak Carnival activities. As an example below, I found tickets departing ORD on Friday, February 15 and returning from VCE on Sunday, February 24, 2019. While that is a long time to spend in Venice proper, you can reach other neighboring destinations like Florence or Milan easily by train and avoid all the large tourist crowds during this off-peak season. Or mix in a ski holiday in the Dolomites where mountain resorts are about 3 hours away by car.
Need a room with a beautiful view of the Grand Canal? Here are all the available hotel deals for the mentioned dates on Hotelwatchdog.