We've learned to pack for three days fewer than the actual vacay - rewear and do laundry once - makes it much easier. It's the extras that take the space. For formal night, we dress like a nice party, not a prom - though some do dress for the presidential inauguration. Semi-formal is toned down a bit and includes a princess dress.
For our precruise days, we treat and pack those totally seperately so we don't have to open more than one bag. That bag also included what will go in the day pack on the first day of the cruise.
All of that being said, I'm usually completely packed about a month in advance - this year I've just thrown things in a suitcase on the side of the room. I'm hoping eventually everything on the list will get checked off.