Ahoy, Patrick!
My husband and I have sailed in the fall for the past two years. We have had "Halloween on the High Seas" activities on both of our fall cruises. Halloween Night was one of the most fun nights we have had onboard a Disney Cruise ship! We wore our costumes to dinner, and then participated in a costume contest in the Adult District later that evening.
In my experience, Halloween night was celebrated in the middle of the cruise both times. Until your cruise gets a little closer, Disney Cruise Line will be unable to confirm which night positively will be your Halloween-themed night. On this night, there were no villains out doing meet and greets, but there was a special "Nightmare Before Christmas" showing and a meet up with Jack and Sally. Characters onboard a Disney Cruise Line ship are always changing, however, so you never know who you will meet!
My advice is to book both your Palo and Remy dinners for nights that work for you. Once you are onboard and learn the week's schedule, if you need to move one of them, I'm sure someone will be happy to accommodate you.
I hope this helps with your planning for your Halloween on the High Seas! Please let us know if you have further questions that arise as your voyage on the Fantasy gets closer.
Happy Cruising, Tracy V.