When I used to host dinner parties, I would plan far in advance. I would tweak a menu to accomodate everyone's tastes and allergies. I would plan how everything melded together, and I would spend the whole time in the kitchen ensuring everything was "just so" and missing a large part of the party.
Now that I am a mum, planning tends to be a few days before the actual party, when I have a spare moment to think about it and jot down a list. Grocery shopping is the night before or day of, and I seem to cram something else last minute into the mix. Inevitably, it seems to come together because the basics are there - good food, good wine and good company.