I really dont think there is a "best day".
Some shows stuff gets sold out Friday, some shows, it leaves with vendors on Sunday. We learn what kind of show it was Sunday when we pack up.
I have a doctor appointment downtown late friday morning, so I plan to stop on my way back and get in line early. There's one thing I'm trying to find that seems to be a unicorn, and if it shows up it probably won't last long.