For most students, we recommend taking the SAT or ACT in the spring of your junior year (either March, May, or June), and possibly taking it again in the fall of your senior year (October, November, or December). Although most students who prepare for their first exam do not significantly improve on a second try, if there is significant evidence that you have not performed as well as you should have (i.e. your practice test scores were much higher), then this schedule gives you a second chance.
Also, we recommend considering both the ACT and the SAT as viable undergraduate admissions exams. You can read about which test may benefit you more in the post entitled Should I Take the SAT or the ACT?
On another note, we do not recommend taking any admissions exam more than thrice, and even that is one too many for most. These exams are intended simply as indicators for college admissions, and do not deserve to be treated with the respect and fear that so many students feel when thinking about them. We advocate taking active control of your test-prep strategy and preparation, so as to avoid the standardized-test-anxiety that is common among many high-school students and potential undergraduate applicants.
We like to think that these tests are merely stepping stones on the path of your future, not turning points. Although they compose a significant portion of colleges’ admissions decisions, a considerable part of your performance on the test can be determined by you, and you alone. Hard work and knowledgeable preparation techniques can enable you to maximize your personal effort on this should-be-insignificant exam.