As mentioned in the introduction section, there were 2 stamps issues the same day (16 April 1958). Only covers with the Expo 58 stamp will be shown here except the last one which I was not able to find with only the Expo 58 stamp.

There are official Expo 58 cachet cancelled during the first day of issue (16/04/1958), and so-called private FDC cachets with the same cancellation day.

Official Expo 58 FDC cachets 

The first two have the same cachet but have been printed in an obvious different blue color. It is not known which one is rarer than the other.

Also a FDC Cover which also has been used for the Luxembourg National Day (19 May 1958) often. There is no trace how this cover was used by both cancellation. The cover was however printed by Campo Rodan and so it could be that the cover was reprinted for the second happening based on the design of 16 April 1958:

 Two different private FDC covers and limited or numbered. The second has the same similar design and editor as the one used for the Luxembourg Day 19 May 1958 - see under Belgian Cancellations > Countries > Luxembourg