However, the best time to take the ACT depends on a variety of factors: the schools to which you're applying, your application deadlines, your cash flow and your personality.
The ACT is offered six times in a year (see the ACT dates): September, October, December, February, April and May.
If you're a senior applying early action or early decision, you'll want the September exam. Scores from exams later in the fall may not reach colleges in time. If you're applying regular admission, you still don't want to put off the exam for too long -- pushing the exam too close to the application deadline leaves you no room to try again should you fall ill on exam day or have some other problem.
If you're a junior you have several options. One is simply to wait until senior year -- there's no requirement to take the exam junior year, and taking the exam more than once doesn't always have a measurable benefit. If you're applying to one of the country's top universities or top colleges, it probably is a good idea to take the exam in the spring of junior year. Doing so allows you to get your scores, compare them to the score ranges in the college profiles, and see if taking the exam again in senior year makes sense. By testing junior year, you have the opportunity, if needed, to use the summer to take practice exams, work through an ACT preparation book or take an ACT prep course.
With all the pressure and hype surrounding admission to highly selective colleges, some students are taking a trial run at the ACT sophomore or even freshman year. You'd do better putting your effort into earning good grades in school. If you're desperate to know early how you might perform on the ACT, grab a copy of an ACT study guide and take a practice exam under test-like conditions.