Most high school students take the ACT or SAT—or both—during the spring of their junior year or fall of their senior year. It's important to leave time to re-take the test if you need to further increase your score before you apply to college.
The ACT exam is offered nationally every year in September, October, December, February*, April, June, and July*. Click on the link and scroll down for ACT test dates.
*No test centers are scheduled in New York for the February or July test dates. No test centers are scheduled in California for the July test date.