Last time I went Halloween night was 2011. It was dead. I remember that I was originally supposed to go to a party with my wife but managed to sneak around it and go to HHN with some buddies...the point being, we got there sort of late (9 maybe?). I didn't expect to get every house done but we did every single house, a couple of them twice, and walked through each scarezone. It was a really, really fun night. I would expect that most people are out partying and drinking, which leaves HHN kinda dead. It's awesome
They didn't do anything special Halloween night in 2011 that I can recall, but in 2009 they opened up a new scarezone on Halloween which brought back all the major icons from the past and put them in Sting Alley. I believe it was to sort of test guest reactions because the 20th year was the next year and, as we all know, they brought all the icons back. However, that's the only "special" event that I'm aware of. Apparently the last show of Bill and Ted is special but I wouldn't know.