I start slowly packing two weeks before. If I don't start gradually getting things together ahead of time, I will go into panic mode the morning we need to leave and end up forgetting half the things that were on my packing list (let's talk about the trip to St. Louis when I had to buy socks because I packed double the amount that my then-two y/o son needed, and none of my own...). So two weeks before, I start slowly packing, and things that can't be packed that early start getting gradually moved to a couple of central locations in the house, so that when I am able to pack them, they're all in the same place.