Speaking of 2600, their news blog, 2600 News, reports that Friday, June 1, 2012 is the 25th anniversary of their first meeting:
It was Friday, June 5, 1987 when we held the very first 2600 meeting at what was then known as the Citicorp Center in New York City. The meeting was modeled after the weekly "TAP" meetings which had been taking place in New York for many years, organized by the phone phreak newsletter which published from the early 1970s to 1983. We had a number of people turn up and it was decided that the meetings should continue. They eventually would expand into cities all over the world and the first Friday of the month became known as "2600 Meeting Day" amongst hackers, law enforcement agencies, and corporate boards everywhere.