Hi there, Thomas!
On special event nights - such a Mickey's Not-So-Scary Halloween Party and Mickey's Very Merry Christ,as Party, the Tomorrowland Terrace Dessert Party will not be offered. There's so much to do on those hard-ticket nights that Guests find it a challenge to sacrifice that much time to any given event!
As for reservations? I'm of the mind that you always protect yourself by having a meal time and place "in pocket" before you head to the Parks. There is no guarantee that attendance at a special event will be substantially lower than a "regular" daytime, so to ease your own mind, it's better, I think, to have those reservations squared away.
Give a call to (407) WDW-DINE, or
book a table online 180 days out.
Have a wonderful - spooktacular! - time!
See you later, celebrator,
-Deborah B.