Well, it depends on where you are going, what you are doing, and with whom you are going.
We do Christmas at a hoity-toity resort with my boyfriend's "adoptive mom" and her family. Everything has to be done by the rules of etiquette, as interpreted by a rather well-to-do group. This pretty much means never being seen wearing the same thing twice - about all I can re-wear is shoes, belts and outerwear. So, before we leave here, I will know exactly what I will be wearing at any given time on any of the four days we'll be gone.
Besides toiletries, laptop, a couple of books, a couple pair of swimsuits, and other personal stuff, I will need to pack:
a) trousers, shirt, tie and jacket for dinner Friday
b) jeans and a nice shirt for Saturday before lunch
c) dressy casual trousers and shirt or sweater for lunch on Saturday
d) jeans and a nice shirt for Saturday after lunch
e) trousers, shirt, tie and jacket for dinner Saturday night
f) jeans and a nice shirt for Sunday before lunch
g) dressy casual trousers and shirt or sweater for lunch on Sunday
h) jeans and a nice shirt for Sunday after lunch
i) trousers, shirt, tie and jacket for dinner Sunday night
j) jeans and a nice shirt for Saturday before lunch
l) dressy casual trousers and shirt or sweater for lunch on Saturday
l) jeans and a nice shirt for Saturday after lunch
For those who are counting, that's 6 pairs of jeans, 6 pairs of dressy trousers, 3 jackets, 3 ties, and a total of twelve different shirts/sweaters! Oh, and I have to take two different topcoats (blue and black), to match the different outfits.
So, yeah. I just pack what my boyfriend tells me to pack!