Each year, SAT is given on one of the Saturdays in October, December, February, March or April, and in June. If you are unable to take SAT on a Saturday for religious reasons, there are special Sunday administrations of the test. The same is also true for ACT.
Since the reason you are taking the tests is to complete an entrance requirement for a college or university, you should take the test in your junior year of high school. Testing in March will give you plenty of time to complete any necessary school work and (if you decide to do so) will give you a few months' time to study for SAT. Then, you will have your results for both tests in plenty of time to apply to many colleges and universities during your senior year. You shouldn't take either test before March of your junior year, as both tests cover the entire high school curriculum.
If you plan on taking either test again, do it in October or November of your senior year. This way, you'll still have time to meet college application deadlines for fall of the next year. The only reason you may want to take the test before your junior year is because you are a middle school student who is taking the test to try to get admitted to a summer academic program.
