Novice days are great for first-time autocrossers because they are designed to familiarize you with how the events work, rather than expect you to know already like a normal event. They'll usually have some "classroom" type session followed by in-car review of your run through the course, followed by some constructive advice.
I haven't been to an Atlanta novice day yet, but I'd imagine it would be helpful for even past autocross participants to identify any bad habits you may have picked up on your own. I'd probably be taking advantage of this event if I wasn't going to be busy moving...
Note that ATL events now ALL require online preregistration! Don't show up expecting to race the day of if you didn't preregister!