For all my Juniors this year, I highly recommend everyone take the SAT or ACT this year instead of waiting for senior year to approach.
Why do I recommend taking the SAT or ACT early? I recommend taking it early because when you first take it there is no guarantee that you will score high, or get the score you need to attend the college of your choice. If you take it early you are able to see where your scores are and that will help you determine if you need to take an SAT or ACT preparatory class or if you need to come up with a daily study plan.
I also do not recommend just taking the test once, even if you achieve high scores that your college of choice wants. Keep taking it and score even higher, so colleges will give you scholarships to attend their school.