Day one isn't what you should be worring about, it's check ride day. That could come on day four through day nine. Memorize the gear down before landing check list and the engine failure below 3000 check list. Set up for clean and dirty configuration, and the associated manuvers required for the private multi check ride. That Seminole supplement is gold but you must stick to it to the letter or you'll have problems down the line. Know by heart all that is contained within the supplement and you'll have no trouble with your check ride(s).
To make life easier down the road at ATP get ALL of your writtens done before you start. Durring the instrument portion you are streched just learning to fly the airplane and studing for the oral, studing for and taking the written is one step you can do with out at that point. Once through the instrument phase the cross country flying takes up the whole day leaving you with no study time. What will end up happening is that you'll have a couple of days in between the X/C and the commercial phase to get your commercial written and instructor writtens done. So no matter how cramped for time you think you are now it is nothing compared to life after October 13th.
I saw people get sent home for not having writtens done, some struggle and get stressed out failing a couple of times before finaly passing. I was lucky and was able to knock out all the writtens before I got there. I might not have faired as well otherwise...
Oh yeah, the most important thing on day one? Make friends! The people are the best part about ATP.