When I was quitting on my own (without AA) 30 days was a common slip-up point for me. For me at least, quitting in the beginning is the easy part because the pain of the last drinking episode is still fresh. When you get to 30 days, your mind and body starts feeling good, and it's at this point when the addiction starts to try and convince you that you are normal or that it wasn't so bad the last time you drank.
Just use it as a learning tool. Now you know that you have to be extra vigilant around the 30 day mark. I think we have to be extra cautious at those milestones - 30 days, 60 days, 90 days.