Cancelling Your LSAT Score
In general, everyone must take the LSAT when they apply for JD. The LSAT score is one of the most important factors in law school admissions.
You may only take the LSAT three times in any two-year period. If you feel not happy with your performance on the test, its better to cancel the score, if you have time to prepare for another test. However, keep in mind that in order to perform your best, you must have a lot of time to prepare in the months prior to the test. If you do not have time to do this, then its unlikely that you will improve. Also, keep in mind that very few people actually feel very good about their performance. If you had a lot of serious problems on the test, then likely you should cancel if you can take it again, but a generalized bad feeling should be put into context.
Finally, if you do not have time to take the test again, its better to receive a score rather than never know, you might be pleasantly surprised. One bright spot is that these days the most or all schools will focus on your highest score, as the rankings now use high score rather than average. However, the schools will see that you canceled your score.
October is fine to apply in the same year, but December is usually a little late. This does not mean you can't get an offer taking December exam, but its certainly not ideal. February is a bit of a gamble; traditionally many schools wouldn't even accept a February score for that year's admissions, but these days there is a lot of flux and uncertainty in the marketplace.
If you decide to cancel your score, there are two ways to do so. You can cancel when you are taking the test or you may send a written cancellation request to LSAC within six calendar days of the test.
If you've never canceled before, doing it once will not adversely affect your admissions chances. Admissions officers realize that everyone may have a bad day, have gotten ill, or had something else affect them adversely, and that it is arbitrary to include such a factor in their admissions evaluation. So, while a cancellation is not ideal, it will not make or break your application. However multiple cancellations will not reflect positively on your ability to set goals and more directly towards them, and will raise the possibility that you are trying to gain an advantage over others by taking the test repeatedly.