I also like the idea of minor holidays, things like vampire emancipation day; remembering when sim vampires were accepted into society (which they obviously were at some point or they'd be hiding it all the time) or maybe Growfruit day; remembering when the tasty Growfruit where first discovered.
I do hope though that holidays don't work like they did in 3, when you had to have the day off work. I want sims to be able to opt out of celebrating a day and be able to go to work as normal.