Swissair Flight North, 15th - 21st May 1939

We have some boxes full of stamps and old letters, coming from the family of my grandmother. Too much to sort out, no idea if the content has more than emotional value or not. Here a philatelist example from 1939, and what I found about it on the internet:

In order to publicize the 1939 National Exhibition, and to send a message to their German and Italian neighbors that they would not be intimidated into submission or subordination, Swissair flew three special round-trip flights from Zurich. These flights were known as the EUROPAFLUG.

The first one, known as EUROPAFLUG / SUD, flew from Zurich, on April 29, 1939, to Rome, Athens, Cairo, Sofia, Bucharest, Belgrade, Budapest, and returned to Zurich on May 6, 1939, the opening day of the National Exhibition.

The second one, known as EUROPAFLUG / NORD, flew from Zurich, on May 15, 1939, to Warsaw, Kaunas, Riga, Tallin, Helsinki, Stockholm, Oslo, Copenhagen, Berlin, and returned to Zurich on May 21, 1939.

The third one, known as EUROPAFLUG / WEST, flew from Zurich, on May 22, 1939, to Rotterdam, Brussels, Antwerp, London, Paris, Barcelona, and returned to Zurich on May 28, 1939.

As mentioned our letter is from the flight NORTH and did the whole trip with the Swissair plane. The letter in the blip is franked with Swiss stamps and canceled with the Exposition Pavilion postmark on 15th May 1939. Canceled again with a ZURICH receiving postmark on 21st May 1939, marking the termination of the trip.