It has been celebrated since 1630 at the time of the black plague in Venice
the Doge made a vow to built a grandiose church (Santa Maria della Salute) and celebrate it every year since, if Venice survived the disease. Estimates provided at the time is that almost half of Venice population was killed during the plague.
still today Venetians celebrate with a temporary bridge, processions, candlelights and more importantly, inviting the family for a traditional dish (eaten only that day) called Castradina.