I have been on NCL and Carnival, but never Celebrity, on Halloween. Both times, there was nothing in the cruise documents suggesting a Halloween party on board. However, passengers brought costumes ranging from the perfunctory to the elaborate. On both ships, the costumed folks congregated in the disco for a party. I "think" it had been announced in the daily program, but I'm not sure.
Anyway, if I'm ever cruising on Oct 31, I'll bring a costume.
I see from your itinerary that you're in Aruba the day AFTER Halloween. I was in Aruba on Halloween once, and yes, they do celebrate it there.