The Happy Haunts Ball will be held on Oct. 29 this year. It will be held in the evening at the MK. It is a seperate ticket event and tickets are limited.
Last year was the 1st year for the event and I did not attend. From what I have read it was limited to 999 people. The tickets were around $120 per person. Once inside you are able to purchase limited edition HM related merchandise such as pins, T-shirts, and other collectibles. Your ticket price included a buffet meal and (I believe) 2 alcoholic drinks. Only a few rides were open: of course HM, SM, BTMRR, and I think CBJ. There may have been a couple of others that I don't remember.
The tickets went on sale in May or June of last year. It is not really a family event.