You will most likely take or have taken the PSAT and/or PLAN in the fall of your junior or sophomore year.
When you receive your scores would be a good time to start planning for the SAT and/or ACT.
When to take the SAT or ACT?
Winter or spring of junior year is a good time to take the first test — as soon as you feel ready to do well.
Many juniors take the SAT for the first time in March, and the ACT in April. Some start earlier or later. Since the ACT only reports your scores to colleges for the dates you specify, some students take a “practice” ACT early junior year.
Keep in mind that you may wish to study more and retake the exam, so factor that into your planning.
Check out the SAT ACT Testing Dates 2011-2014 calendar to see when the tests are given along with the registrations deadlines.