Today (April 25) is San Marco's feast day and for centuries, this day was celebrated by Venetians with grand processions and pageantry in honor of St. Mark. It's still celebrated today, at least in the Basilica, where the Patriarch celebrated Mass at 10:30 this morning in honor of St. Mark the Evangelist and Patron Saint of Venice.
I don't know if there are still any other festivities - I checked the Piazza San Marco webcam this morning, and it looked like a normal rainy spring day in Venice. Checked it again later, and the sun was out, the orchestras were playing, and the scaffolding around the campanile looked to be growing.
April 25 is also the Festa del Bocolo (Festival of the Blooming Rose) where it's traditional for Venetian men to give a red rose to all the women they love. That could get expensive for some guys! There are a couple of legends associated with this tradition retold (and charmingly translated) on Venice Explorer.