Assuming that all the prerequisite courses have been taken and diligent study habits are in place, the best time to begin preparing for the SAT or ACT is at the beginning of your junior year.
First, if preparation begins in August of your junior year then you have several weeks before school starts to focus exclusively on your SAT or ACT preparation.  You will then have all of your junior year to maximize your test scores.  Remember, since the colleges take the best composite score once you have taken the SAT or ACT the score can only go up.
The worst time to take the SAT or ACT for the first time, in my experience, is during the spring of junior year.  This is when students are the most burned out and sleep deprived, which never works well on these very difficult tests.  Furthermore, this time of year is when AP and final exam preparation are at their peak – leaving students little else to focus on.
Now, let me make what I am saying more clear.  Should a student never take a Spring SAT or ACT?  That is not what I am saying.  If you start early in October to January of the junior year then the preparation for the Spring test is much less intense as long as the student keeps up and gets sufficient sleep.
Overall, I would say the best times to take the SATs and ACTs are September/October, January/February, and June.  Those seem to be the best times when students seem to be generally less stressed and the results seem to be much better.

Steve Kirshenbaum, M.S. is an Educational Consultant and Founder of SAT Preparation Group, which specializes in worldwide ACT / SAT coaching. SAT Preparation Group advises in test prep, college planning, and success strategies for teens. SAT Preparation Group can be reached at 877-672-8773 or www.SATPrepGroup.com.
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