I use the 21 day seasons mod and try to let the holidays be roughly where they should be in the month running on 1 week=1 month. So Winterfest is in the first week of winter and New Years at the end of that same week and sims has the days in between off. My holidays in general:
Winter:
Week 1 (December) - Winter's eve, Winterfest, Winter's rest, New Years Eve (All days are holidays, giving sims 4 days free Wednesday-Saturday)
Week 2 - Story day (read books, tell stories, no time off, just something fun)
Week 3 - Love day on Saturday, I like the idea that love day ends winter.
Spring:
Week 1 (March) - Springfest (The equinox, celebrated mid week)
Week 2 - Eggster (Sunday, so first day of the week), Walpurgis night (Saturday, so last day of the week)
Week 3 - Hug parents day (last day of spring)
Summer:
Week 1 (June) - Last day of school (wed), School holiday (thurs), Summer solstice (fri), then summer holiday starts on Saturday for all sims.
Week 2 - Summer holiday for every sim (no work or school), one activity per day such as "water fun", "bbq", "work out".
Week 3 - no holidays.
Fall:
Week 1 - (September) Harvestfest (As a European I think a harvestfest should be at the end of harvest, so at the latest September)
Week 2 - Simblreen (last day of the week, as it is last day of October)
Week 3 - Mourning day (first day), Bon Fire Night (Monday, not a day off)