Strictly judging by how EA has delivered packs so far, I have a strong feeling that holidays will be a combination of systems that we have already. Specifically, holidays may be a hybrid of planning parties and determining club-like activities. As a bonus, I suspect that, from now on, all parties will be able to be planned in advance with the new calendar or by days of the present week.
Because the game has already introduced diversity in clothing, it's possible that we will be able to determine which types of these clothing assets would be appropriate for the holiday -- Indian-inspired, Asian-inspired, Hispanic, etc. From what I've seen of Sims 4 Studio, CAS assets can be tagged by culture, so it seems possible to plan holiday gear from the cultures already in the game or added by a patch.
Anyway, that's my wild guess.
As a bonus, it would be really cool to be able to plan birthdays in advance now. Each of our Sims could have their own "holiday".
