14 and 15 August 1958
Since the Vatican was issuing its own paper money and stamps already in 1958, they wanted to be present to respond to the philatelic collectors.
This was certainly noticed through the different official covers issued (sse below). It is the only country with two different official "non Campo Rodan" covers.
The most common cancellation was the 14 August, however some rare typical philatelic items were cancelled on the 15 August (see below).
Some covers were cancelled with the official cancellation (scan of cover to come) (R)
Official Expo 58 cover edited by Campo Rodan
The Official edited cover with logo of the country (edition is not identified)
Second Official edited cover with Vatican Building (edition is not identified)
Official card with the cancellation of 15 August 1958 (R)
Official cover issued dor the Catlogic Days with German stamps cancelled (R)