It should take place Sunday, Thursday, Friday and Saturday nights that week.
It's often been voted the best Halloween event in the country. It's certainly been one of the best consistently for over a decade. The event changes every year, but you can be certain the most popular rides will be open and there will be around 7 or 8 highly themed haunted houses/"mazes" to walk through--they can be terrifying, I've seen grown men scream and women break down in tears. In addition, there are "scareactors," performers in costume trying to scare you along walkways throughout the park, sometimes even in shops and restaurants. There are also event-specific shows--one, Bill & Ted's Halloween Adventure, is an annual comedy show with a pretty large cult following. There are a lot of videos on YouTube if you want to get more of a feel for the event.
HHN IS NOT FOR CHILDREN. It's gory and frightening, with a ton of sexual innuendo and alcohol for sale every 100 feet. There's a continuing debate about what the minimum age to bring a child should be--I've rarely seen anyone argue under 13, I would go as high as 18 personally. BTW, costumes are not allowed.
This is a separtely ticketed event, like Mickey's Not So Scary. (Ticket prices are the only times these two events should be mentioned in the same sentence.) Prices vary depending on the day of the week, and there are all sorts of discounts available, but figure the $45 - $80 range. Multiday tickets area available and offer a huge savings.