I'd have to know for sure how the holiday system would work but as of right now Id make
-Birthdays
-Family day (A Canadian holiday) I think in rl this day is useless but the sims need family day. My sims never spend time with family outside the current played household.
-Heritage day (?) on this day all the ghost and past family members will all come together in one house and meet the new generation of family and give the head of the family a gift/inheritance/blessing. Considering all the dead family members always invite my sims out to dinner and bars (Im not sure how cause there dead) they can now all socialize to there ghost hearts content. Kinda like in Mulan lol.