It depends, but I find that if I pack too early everything just gets very wrinkled. I don't have time usually to iron everything to a crisp.
If I am going in the summer, I tend to mostly pack over a day or two- as I wearing most of my favorite summer clothes. In the winter months , I tend to pack a few days earlier. I may dig out items a month before the trip, try them on. Go shopping to cover any needs. I wash them within a week before I go and keep them in a group on hangers.
I do gather some stuff ahead of time, and make a list. I also sort of keep certain toiletries in a Ziploc all the time: travel tubes of toothpaste, travel size mouthwash, sunblock. I also have a few things that are more or less WDW stuff: like temp tatoos, leftover Mickey stickers. Somehow they sort of just stay in the suitcase since I don't need them.
A certain number of things, like my cell phone and charger, favorite hairbrush, I find that I can't pack until the night before or day of the trip.