I'm doing same cruise in October and trying to plan as well. It looks like separate nights, but as this is a new itinerary and Fantasy only did a few 4 days Halloween cruises in 2021 right after cruising resume, who can say. I like to reference the
Disney Cruise Line Blog personal navigators. This is closest I found on the Wish last year. I would guess Trick or Treating and main halloween stuff on the Nassau day and Pirates on
Lookout Cay day. I would hope for something Halloween on the sea day, but on the Wish itinerary, nothing is jumping out at me on the actives list. Note the Oogie Boogie Bash 18+ on the Wish is unique to just the Wish and they had it during Christmas as well, its not special for halloween.