i hafta ask?
If you're asking how and why, I do not know. One of us, it may have been me, posted it for the first time waaayyyyy back on the old Air Warrior BBS at least 10-15 years ago. Since then, every June 6, someone posts "John has a long mustache", on at least one BBS or news group somewhere, and the other code phrases follow.
At one time, there were all sorts of things done like that. Posts would be made on the anniversary of various events. On the anniversary of the real "Great Escape", lines from that movie were posted. For another event, lines from "Stalag 17" were posted. The only one that really stuck was June 6, the others came and went. Back in the day, there were a lot of hardcore history enthusiasts in the community who were also hardcore players. We even had a few real World War II pilots and crew members in the mix.