When Should I Take the LSAT and an LSAT Course???
It is often advised that it is ideal to take the LSAT in the spring or summer of your junior year (assuming you plan on applying to schools in the fall of that year). If you take the June exam, you are in a good position to apply early in the fall to start school the following fall. If you do poorly, you can take the October exam and still be in a good position. Many students decide to take the LSAT late and will wait until the last minute to take the exam. The February test is the last exam you can take and still apply for /some/ schools that year.
You want to take the test at a time in which you are able to dedicate the time to prepare. Hopefully you will be in a position to avoid the test when it falls right around finals. Most students take the test in October and they prep over the summer. This is perhaps the second best time to take the test for many people (depending on where finals land, schedule, etc.) but it is still a good time, as it is not the last minute. You can still take the December test, if necessary, ang get your applications completed in time for fall admission.
You will want to take a prep course that ends close to the actual test date (within around one month). You will not want to take a course in the spring and then take the December exam. The majority of your studying should take place right around the test date (starting a few months prior). This is why most test prep courses end right before the exam date.