The Birthday Ball is one of TG's "big" events at the Coronet (capacity c.2500 people!) with the usual mix of performance, decoration, decadence & playrooms.
Details & tickets at http://www.torturegarden.com/events/birthday/
Tickets will likely sell out at least a couple of weeks in advance, so suggest people buy them ASAP.
A group from here can meet up at a pub nearby for pre-drinks (and to change if needed) then head to the venue around 11pm. If you went to the Valentines event then you'll know the drill already!