snarl seems to be down ATM. But nevertheless I'd say October would be good for a Hackathon. Then there would be about a month to test the code produced at the Hackathon before the 3.5.1 release is scheduled.
October is bad in eLok, but November should work. Now HSV has home games on 6th and 13th which would make the weekend of the 19th/20th the one in November @Michael Vogel would be free (?). What about the rest @Rabuzarus?
Usually I can't say something about a date which is 2 months in the future I know we do planing a PunkRock Festival in November (but the exact date isn't clear at the moment). Form the present point of view 19/20th November sounds good