The SAT is a standardized test meant to show schools how prepared you are for college by measuring key skills like reading comprehension, computational ability, and clarity of expression. Because so many students take the test, it also provides schools with data about how you compare to your peers nationwide.
You'll almost certainly need to take the SAT or ACT if you're applying to any colleges or universities in the United States, since most require you to submit test scores with your application. Depending on where you want to apply, your ACT or SAT score can account for as much as 50% of the admission decision, so a strong standardized test score is vital. Seniors take the test in the Fall and Juniors in the Spring. Below are the 2019-20 test dates.
The ACT is a standardized test designed to show colleges how prepared you are for higher education by measuring your reading comprehension, knowledge of writing conventions, and computational skills and then comparing you with the rest of the high schoolers who take it. It essentially serves as a nation-wide college admission test (though it's far from the only factor schools consider).
Most four-year schools require applicants to submit either ACT or SAT scores (they don't distinguish between the two), which can then make up as much as 50% of the admission decision. A strong standardized test score is a key part of your application. Seniors take the test in the Fall and Juniors in the Spring. Below are the 2019-20 test dates.