Actually, it's probably a good idea to take it in September, because you can do the bulk of your LSAT prep over the summer. I got very busy in my second semester junior year, and my senior year was completely insane. Getting the LSAT out of the way early will free you up for the school year so you can keep your grades up. Your GPA and LSAT score will be key factors for admissions.
Plus, if you don't do well on the September LSAT, you still have plenty of time to retake without panic. Be aware, though, that most schools average your scores, so prep your butt off for the first one and aim for it being the only one. Take a prep course if you want some extra insurance, so to speak. It seems that most of the high scorers took one before their first LSAT, and those who didn't take one, often wished they had.