Twelve Days of Xmas Trivia

[L Verge]

I am cheating on the sign and asking that you give Bill B. credit also on the answer because he sent me the same email and photo a few weeks ago.

Turn Lights Off sign is posted at the tennis courts on Fort McNair. Those courts are located where the execution of the Lincoln conspirators occurred. I bet the military has not caught the significance and humor in this sign that they posted.