In 2006, Pope Benedict XVI opened the files of the Pontificate of Pius XI (1922–1939). Documents of the archives of the Apostolic See can be consulted up to 11 February 1939, with some exceptions. One such exception are the fascicles concerning the death of Pius XI and the election of Pius XII in the Archive of the Nunciature of Warsaw, in the Vatican Secret Archives. They contain updates from the Roman Curia to the Warsaw Nunciature and official messages of congratulation. Local church archives are invariably open beyond February 1939, and I have translated a message from the Oriental Congregation addressed to Bishop Ladyka, which was sent to all Eastern Catholic superiors. Historians eagerly await the opening of at least a portion of the Apostolic See’s archives from of Pius XII’s pontificate. Perhaps this eightieth anniversary will be the harbinger of that happy event.
Update: On 4 March 2019, Pope Francis announced that he has consented to the opening of the fonds of the pontificate of Pius XII (1939–1958), in March 2020.
From the Archive of the Nunciature of Warsaw: