After you get really intoxicated on your substance of choice, be it 33 or Beer or anything else you care to take; you go into a really dark room or a room with some nice blue flashing lights, you close your eyes and turn on heavy trance music. After a while of really listening to the music your body is overwhelmed by an awesome feeling and you enter a dream state. That is a Head Party - works best with 33.
'Duuuuuude I am WASTED - let's start the head party!'
'Fuuuck me mate! Get some munchies and let's head party'
when one girl is with a group of five or so guys and gives them all head one by one.
"dude, i heard shannen had a head party last night!"