I attended a coupla days of Celebration IV in LA a few years back. I dunno if there really are "best days". The sparse days, Thursday and Friday, generally, you can enjoy the exhibits with less people, but you may miss out the fantastic cosplay, which you never know when they'll show up. One of my fondest memories of Star Wars Celebration IV was when the Cylons (OG chrome costumes) walked through the halls, louding announcing their presence with "woo-woo" eye-scanning sound effects.
The weekends can get tightly packed, and you'll have to line up for everything, but that's when the popular speakers show up. What kind of experience are you hoping to get? Sundays, things die down, but you can get blowout bargains from the stores.
If I were going, I'd check leiasmetalbikini.com, or some other cosplay sites to find out when and where the Slave Leias will gather for pictures.