Typically, on a 4 night cruise (Dream), Pirate night is night 3. But, on our Halloween on the High Seas cruise, they moved it to night 2, with the Halloween festivities on night 3.
My guess is they will do it on night 3 (even if it is Christmas), but it could be on Key West day, since you depart fairly early from there. Although, DCL typically doesn't to special dress nights the last night of a cruise, since people need to pack up and get their bags out by 10:00-11:00 that night.
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!