Are you taking Biology this year? AP English? Algebra II and Trig? If you answered yes to these or any of the thirteen subject areas offered, you should sign up for the SAT II Subject test in May or June.
The SAT II Subject tests are hour-long standardized tests that focus on one topic. Because these are subject specific, you would be wise to plan to take the test during the school year you are studying the topic. You can take up to three exams on one exam sitting.
Many colleges do not require these tests, however, most highly selective schools do require at least two subject tests in two separate disciplines. Schools that do not require these tests will accept these scores and include them in your admissions file.
Take a look at the College Board site to see all subjects offered and find more information on registration and how to prepare for these exams. The dates this spring are May 3 and June 7 2014. Registration deadlines are about one month prior to the exam.