I almost ALWAYS start packing at least a week beforehand!! Heck, we're going to Asia in OCTOBER and yesterday I was all "should I start doing a trial run now?!" BAHAHA.
Packing early is like, another way to get excited, another way of "planning" for some people. I like to use it to make sure I have everything (ie: what do I still need to go out and buy) to make sure everything fits in the right sized suitcase I thought it would, etc. I usually have a "park bag" of things that stay packed, like, cm appreciate cards, pins, tiaras, mickey ears, the kind of stuff that I won't use at home at all, that helps with the process. Sometimes I'll end up taking out things, mainly clothes, to make more room, I won't wear everything anyways!
Then the few days before I'm usually all packed, do some laundry, throw the last few things in and call it a day!