Sophomores enrolled in advanced math are encouraged to take the PSAT in October.
Juniors should take the PSAT in October. They should then take both the SAT and ACT in the spring. Spring sport athletes are encouraged to take the March administration of the SAT.
Seniors should re-take either the SAT or ACT in the fall. Take whichever test you felt more comfortable with. We recommend testing again even if you were satisfied with your original score. Many (not all!) college base institutional scholarships on scholarships and GPA. There are many instances where your existing score might be enough to get you into a college, but another fifty points on the SAT could result in a higher level scholarship. Although spending another $60 to take a test on a Saturday morning isn't exactly appealing, it's a worthwhile tradeoff if it nets you several thousand in scholarship money.
There are some students who want to test early and often. For most students, once in the spring of junior year and once in the fall of senior year is plenty. However, it's OK to test more than that as long as your coursework doesn't suffer. There's only so much time in the day; make sure it's spent on the right things.