I think there is a thread like this but whatever :)
Mine will be the general holidays (gift giving, new year's, spooky day, love day, harvestfest). And like one summer & winter holiday period for everyone of a few days each. I'd also like to do a Forgotten Hollow Founder Day for Vlad, Kid day, Mother's and Father's day and maybe some family-/save-specific traditions. Say if the owner of a popular restaurant dies, they'll come together each year to remember him together.
ETA: actually, since I still want them to have to "take" vacation days, I might do one Family Day instead of Mother's / Father's / Kids' Day where everyone has a day off. A day designed to make the whole family spend time with each other & do activities with the immediate live-in family (given that extended family will already come around for Gift Giving, New Year's & Harvestfest). Also, spooky day, love day & most family-/save-specific holidays will probably not include days off for everyone but just be normal workdays, since they're not "national" holidays so to speak, but the other type of holidays where you still have to go to work :D Cause with Family Day + 4-5 days of winter holidays (which will include Gift Giving + New Year's) + Harvestfest + 4 days of summer holidays, my sims will already have 10-11 vacation days as it is. If the year at max setting is 84 days (at least I think we can set one season to last max. 3 weeks = 12 weeks for a year), that should be enough official time off for everyone. Mostly because I want sims who'd like to go on vacation for a longer period of time to actually have to earn more days off first.