When We Get Behind Closed Doors

Wanna know about the history of secret sessions of Congress? Me neither, but there's always one in any crowd, and thankfully the Federation of American Scientists lives for publicizing otherwise unpublished data chunks from the Congressional Research Service about stuff like this. Sample from the PDF file:

Since 1929, the Senate has held 53 secret sessions, generally for reasons of national security. Six of the most recent secret sessions, however, were held during the impeachment trial of President William Clinton. Two of those sessions were in January 1999 to discuss a motion to end the trial and another motion to call witnesses. Four were in February 1999 during the final impeachment deliberations.