As an example, the 9/11 Memorial and Museum opened 5 years after 9/11. The National World War I Museum opened 8 years after the end of World War I. Was there ever a museum that opened after an even shorter time period following a major historical event?
The definition of an "event": a historical occurrence that wasn't easily predictable in advance. For example, Nixon's Presidential Library wouldn't count as commemorating the "event" of Nixon's presidency because it was entirely predictable that someone will be elected President in 1969 and that this someone will end up having a library.
Update: the definition of "opening" is that a member of the general public was able to attend said museum and see at least one exhibit.