1 July 1958
Pure technically speaking, I should not have to add this country on this website right now, since in a first instance I am only covering Belgian covers or at least covers with Belgian stamps and/or cancellations. However is Belgian Congo is official part of Belgium in 1958, and that is also the reason why I am putting this now in.
The Belgian Congo covers and/or philatelic items are rare to find and also confusing, since you have a mix of date cancels and stamps used.
That is the reason that you will encounter cancellations of 17 April 1958, being the same day of the celebration of Expo 58 first stamps issued on 17 April 1957. See also under "Mysteries".
The second most used date cancellation is 5 May 1958, which is also the one to be found for the special leaflets of the Flowers serie (see also the set of Maximum cards of these flowers), the leaflet of the red cross all sold at the Belgian Congo Building at Expo 58. I suspect that the 5 May 1958 was the official opening of the Belgian Congo Building but I am not 100% sure.
The third date cancellation is 1 July 1958. This date was the official National Day of the Belgian colonies including Congo and Urundi and Ruanda. It is also on that same day that the Belgian Congo is issuing a set of 5 stamps depicting the 5 Belgian Kings. Both happenings was to celebrate the anniversary of 50 years existence of Belgian Congo.
The only cover I found used for many FDC cancellations of Belgian Congo stamps (same cachet) which was edited by Grumiaux, date cancellation of 17 April 1958, also other cancellation to be found, but which are quite rare.
Official maximum cards with the date cancellation of 17 April 1958
Official Maximum cards with the 5 May cancellation:
An official exposition card with the cancellation of 5 May 1958, and with a Belgian Congo stamp. Followed by 15 maxi cards of the flower set.
You will notice the Red Cross Leaflet sold at the Belgian Congo Building with the same cancellation of 5.5.1958.
Finally a special first day sheet of the set of Belgian Kings on stamps of Belgian Congo. The day of cancellation is 1st July 1958, which is the national day of Belgian Congo.
