Since the set of stamps were issued on 18 March 1958, the official FDC covers were cancelled that day. However, since the official opening of the Exhibition was on 17 April 1958, you will also notice official covers with the cancellation of that day.

There are official Expo 58 cachet cancelled during the first day of issue (18/03/1958), and common FDC cachets with the same cancellation day.

Official Expo 58 FDC cachets

Two different types of paper used:

Official FDC cachets (used by any FDC Cancellation

First day covers cancelled in Vaduz on 17 April 1958 are often cancelled on foreign covers and are rare to be found. It is not known if covers were cancelled at the Liechtenstein building at the Expo 58 in Brussels. 

There is one exception about the cancellation on 17 April, being a singe card with the Liechtenstein building depicted and cancelled on 17 April 1958 (see below)