Buying Tickets Early

I see that you can only buy the tickets the day of, but can you go really early in the morning to get for the 10:47am ride just to make sure you get a seat? Will they sell in advance? Also is the 6 yuan for a round trip? Or is it just for there? I actually have been to Badaling once but had a friend do most of it for me so I wasn't really paying attention...I am going with my parents on Sept. 28 - which leads me to my next question...since the Mid-Autumn holiday falls on the 27th this year does that mean the ride will probably be super full on the 28th? Or will everyone be gone by the Monday? Is the Mid-Autumn festival a big deal? Will there be a lot of travel in BJ at that time?