It's important to plan early for standardized college testing so that you can focus your efforts on applying to colleges based upon your scores. During the sophomore year (or the summer before junior year), it may be helpful for students to take a diagnostic test to determine whether they should focus on the SAT or ACT. All colleges accept either test.
Based on the results of the diagnostic, juniors should take either the SAT or ACT in the fall/early winter. Ideally, juniors want to have one or two tries with the SAT or ACT by mid-spring, so they can focus on AP exams, finals and possibly SAT subject tests (students may take up to three subject tests at one time). The early fall of senior year is an opportunity to repeat the SAT or ACT to try to achieve a higher score.
For SAT testing schedules and registration deadlines, check out the College Board website. Khan Academy offers free online SAT prep. For ACT dates, ACT Inc lists tests and registration deadlines. ACT Academy is their platform for free test prep. Students may also want to check out a list of test-optional colleges at Fair Test.