The Festa del Redentore: Dinner & a Show!
To tell you the truth, not all Venetians do the procession across the votive bridge to the Church of the Redentore on the island of the Giudecca every year... but that doesn't mean they don't celebrate! (In fact, it would be inconceivable for a Venetian not to celebrate this festa!!)
As you can see here, Saturday afternoon and evening, everyone who can beg, borrow or steal a boat decorates it, piles in family and friends, and heads out to the Giudecca Canal and the Basin below Saint Mark's Square. Then everybody, whether they're in a boat or not, has a massive feast of traditional Venetian dishes which lasts for hours, and waits for 11:30 pm, when the fireworks begin!
For a good 45 minutes, fireworks are fired into the Venetian night sky from the Giudecca Canal, and make quite a spectacular effect framed among the ancient bell towers of the city and reflected in the waters of the lagoon, as you can see at the link above as well as at this one!
As an experience, I highly recommend it (especially in the company of festive Venetians!)
(You can read more in English about the festivities, and see more photos like the spectacular one at left, at Venice's official city website.)