Hi Kathie,
Most advanced dining reservations at the Walt Disney World resort can be made 180 days in advance, but the
Tomorrowland Terrace Fireworks Dessert Party is an exception. Reservations for that special event aren't usually accepted until closer to the date of the party.
For example, reservations for the dessert party in early May opened just last week.
The good news is that you should know the dates of Mickey's Not-So-Scary Halloween Party well before reservations for the late October dessert parties open. The bad news is that I don't believe the party is staged during the Halloween party, and, because reservations do not seem to be released on a regular schedule, you will probably need to check back regularly to try to secure a spot on available nights.
I haven't been fortunate enough to participate in the dessert party yet, but many of my Moms Panel friends sing its praises. I'll put it on my "must-do" list for this year, too.
Good luck! Please let me know if I can answer any other questions, and enjoy your visit!
Wilma