Monthly Maths Jam
The story goes that Matt Parker (http://www.standupmaths.com/) was having a pint with some friends in a pub in London one evening (Tuesday April 29th, 2008) when someone mentioned a maths problem they were working on. Napkins to the fore, brows furrowed, and much scribbling took place, with someone even pulling out a laptop.
History doesn't relate whether that original problem was solved, but the the story goes that conversation then ranged widely and wildly across other maths problems and puzzles, and the time just flew.
At the end of the evening one participant said "This has been fun, we should do it again," and another participant who might perhaps be described as more "detail oriented" pulled out their diary and said - "OK ... when?"
So over the next few months they and others got together, had a refreshing beverage, and did maths. Recreational maths such as puzzles and games, as well as "proper" maths, and just generally allowed themselves to enjoy a subject that usually has to remain hidden, shunned by the wider population.
They settled on the penultimate Tuesday of the month, as that's (a) likely to drift in and out of phase with any other regular commitments, and (b) weird.
And from there it took off. Monthly MathsJams now happen at 70 to 80 places all over the world, and is going strong. You can read more about them here: