The Nationals Archives is hosting a series of events for the 50th anniversary.
A tip from someone who has been there.
As their press release notes,
Attendees should use the Special Events Entrance, located on the National Mall at Constitution Ave. and 7th Street, NW.
This is a bit confusing because the entrance is not on the Mall.
Go to this map. Where it shows the exit on the Constitution Avenue side, that is the Special Events Entrance. They usually bring out a sign, but may not always.
Also note there is absolutely no photos allowed inside, but once you are in the auditorium, you can photo the event.
There is a cafeteria downstairs, but it is small. They also have a nice book store.