It's a gathering of great developers eager to hack on new/other technologies than they use in their day job. It's a room to explore, learn and collaborate. Just like the photos show, we meet up have pizza, teams form and work on ideas, and at 6pm we open beers. btw., it's free.
Plus, it's only half the day off on friday, right?
I would have signed up anyways but i can't see taking a day off from work not really knowing what i might run into.