The LSAT is administered four times a year, in June, late September or early October, December, and February. The February test date is usually too late for most law school application deadlines, so it should generally not be considered as an option. That leaves summer, fall, and early winter test dates.
If you can prepare for the LSAT throughout your junior year and take it in June before your senior year, you will be informed of your scores during the summer and, therefore, have the opportunity to determine at which schools you will have competitive edge for admission. However, you should not take the LSAT unless you are prepared! If, for whatever reason, you feel that you are not well prepared to take the exam in June of your junior year, by all means, take the entire summer to study intensively and then plan to take the fall LSAT, which will still allow you time to determine appropriate schools and submit applications in late November/early December.
The December LSAT should be a last resort. Unless there have been extenuating circumstances that prevented you from taking the June or September/October test, you should not wait to take the December exam. There are two main reasons not to wait until the December test date. First, your scores take approximately six weeks to get back to you. Since LSAT scores and undergraduate grade point average are the two most important factors in the law school admissions process, you will be at the distinct disadvantage of not knowing your LSAT in the summer or fall. The other reason you should not wait is that you may want to retake the LSAT in December if your June or September test scores were not where you know they should be. Although we did say you should plan to take the LSAT only once, it is always wise to have a back-up date, just in case.