The ACT is a college readiness assessment that students across the nation take in their junior year of high school. It serves as a college admissions and placement test. The state of Illinois provided a free ACT test to all juniors for over a decade and it has served as the primary college-entrance exam for Illinois students. Beginning in the spring 2016, as a result of a change in state policy, Illinois high school juniors transitioned from taking a state-funded ACT during the school day to taking the SAT instead. Students interested in registering for an ACT on a Saturday national testing day should see their counselor for more information.
More information on the ACT can be found here: http://www.act.org/content/act/en/products-and-services/the-act.html