No it's a reference to one of the older Forum incarnations (I believe it's the Forumplanet one).
It's refers to a date which was I guess sort of a spam attack, but ended up being very non-effective, but it warranted a rememberance. Sort of a forum inside-joke holiday.
The reason it's 7|15 is because of the similar 'planned' version of the attack, to the 9|11 NYC ones... only unlike those, this one was mostly averted.