I have never been on a Halloween on the High Seas cruise. My best suggestion is to look at Navigators (can be found on the Disney Cruise Line Blog) for Halloween week sailings last year, and check out what was happening on Halloween day.
We sailed over Halloween day back in 2013. I've also done other HOTHS cruises. There wasn't anything different on the cruise than the ones where Halloween was just celebrated on a particular day instead of exactly on October 31.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!