Anniversaries, big birthdays, retirements, post-divorce, post-something. A specific date carries weight. The trip has to be capable of carrying it.
Milestone trips fail in a recognisable way. They fail by being indistinguishable from any other trip the household has taken. The dinner is good. The hotel is fine. Nothing in the shape of the days signals that this trip is the one marking the thing it's supposed to mark.
The work is in identifying the defining moment and building everything else around it. A sixtieth in Mustique with the whole family flown in two days before, secret. A fortieth anniversary at Aman Tokyo with a specific room booked twelve months ahead. A retirement that begins on the last day of work with a private jet to Cape Town. The moment matters. The moment has to be earned by the rest of the trip.
Milestone trips usually run ten to fourteen days. Sometimes shorter, because there is something to be said for a deliberately short trip that does one thing properly. The properties carry weight here in a way they don't on sabbaticals; one wrong hotel can damage the whole occasion.
Once you've had one designed properly, you understand what was wrong with the previous five.
The other five
When you're ready