There are still a few Multics sites running - and semi-active community development.
Courtesy of an old fork of SIMH that became a very full fledged DPS8/6800 simulator. I was running MR11 something on a Pi for a while, and then in GCP.
Brought back old times - my second computer - after GCOS.
*sigh*
I think there may also be a reference to Flag Day in the original printed Hacker's Dictionary, and I think in the Jargon file from which it was derived? I don't have any old copies of either around to check
--tep