I'm not sure whether you are a TCA member or just attending as the "general public" so I will break some stuff down for you to get the best experience on Saturday.
If you are a TCA member, and you want to try and get everywhere on Saturday, bring your running shoes. I would highly suggest getting there at 9:00, right when they open. Go to the member halls first (silver, blue, red, and white). A lot of the vendors in these halls will slowly start packing up by 10 or 11 and will be out the door by noon. Then go to the dealer halls (Orange, and Purple). They're usually the last ones to start packing up. Keep in mind that the hours for the dealer halls are now 9am-3pm, probably because a lot of the guys in these halls start packing up at 1 and leave by 3 since there are not a lot of people there Saturday afternoon. I would forget about going to the black hall where the modular layout is if you're just looking to buy trains.
If you're attending as the "general public" I was just stick to the dealer halls because if you register to get into the member halls on Saturday, chances are you'll only be able to see a fraction of what's there since most of the vendors will be gone.
Regardless, if Saturday is the only day you can attend, go for it. There are still plenty of trains for sale and you may not get everywhere since there's so much to see, but it's still a fun time.