In Venice the feast day of Saint Roch on 16 August commemorated the end of the terrible plague of 1576 (in which Titian died). On this day the Doge would hear mass in San Rocco where Saint Roch was buried, to celebrate his intercession in bringing the plague to an end.
Canaletto's painting shows the grand procession of state dignitaries and ambassadors emerging from the church. The participants all carry nosegays, which were presented to them on arrival as a memorial of the plague. The Doge carries a parasol and wears gold and ermine ceremonial robes. Awnings give protection from the sun.
An exhibition of paintings was traditionally held at the Scuola di San Rocco, which dominates this painting, on Saint Roch's feast day. The pictures decorating the Scuola here have not been identified.
