I'm looking forward to making up fictional holidays. I really want more Halloween style celebrations so if it's a versatile system, my Sims will be playing dress-up a lot, lol. I'd also like to honor Dia De Los Muertos (Day of the Dead) permanently in my game because I just loved the event that one year we had it. If I can, I think I might have some supernatural holidays to honor Vampires, Aliens and Ghosts, and I might make a creepy Lovecraftian holiday too, for cults who worship the "Ancient Ones", and a day to honor Edgar Allen Poe, like "Day of the Raven".
I'm really looking forward to seeing how the custom holiday feature works. :)