On 12 January 1953, he was appointed Patriarch of Venice and, accordingly, raised to the rank of Cardinal-Priest of Santa Prisca by Pope Pius XII. Roncalli left France for Venice on 23 February 1953 stopping briefly in Milan and then to Rome. On 15 March 1953, he took possession of his new diocese in Venice. As a sign of his esteem, the President of France, Vincent Auriol, claimed the ancient privilege possessed by French monarchs and bestowed the red biretta on Roncalli at a ceremony in the Élysée Palace. It was around this time that he, with the aid of Monsignor Bruno Heim, formed his coat of arms with a lion of Saint Mark on a white ground. Auriol also awarded Roncalli three months later with the award of Commander of the Legion of Honour.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): When did he take up his new diocese in Venice?
15 March 1953