If I could pick only one day, it would probably be Friday (the day with the most meet hours available). Either Thursday or Friday are fine. Saturday is pretty much a wrapping-it-up afterthought for both buyers and sellers, but has the advantage of significantly diminished crowds.
However, it kind of depends on what you might be looking for. If there is some especially hard-to find item on your list--something being actively sought by others in the hobby as well--you're probably better off getting to the meet at its earliest stages, and that would be at noon on Thursday.
