The Law School Admission Test-LSAT evaluates applicants in the areas of reading comprehension, critical reasoning, logical analysis and writing. The 3 ½ hour exam is the single most important element in your application. At most law schools it is weighed more heavily than your GPA. This score will also largely determine the level of any financial aid you receive.
The exam is offered in February, June, October, and December. While the October (and less so, the December) exams are fine as far as the timing of the submission of the application goes. Take the LSAT when you are ready, but if possible make plans to take it in June after your junior year. Note that the February exam scores arrive too late for most, but not all, law schools.