National Void Critter Day, sounds like a fun idea!
Think there's more threads like this over in the Expansion Pack category but I suppose it works here too.
I had not realised until recently that scheduled events weren't the same concept as scheduled holidays, so after work game nights should work really well. Personally I'm most excited for wedding planning.
I've pondered making Book Day as a holiday, I don't know if it's something that only happens where I live in Wales, but one day of the year kids dress up as their favourite book characters and celebrate authors. (I'd probably make some fictional famed authors/books in my new game.) I'd also have the adults from my book club celebrate it as well though, and any other bookworm Sims.
I mean, I haven't got any holidays set in stone yet as I'm waiting to see all of the traditions before me so I can really pin them down.