Both my D had at least taken either SAT or ACT once in the first semester of junior. I strongly suggest students to finish taking ACT/SAT by the end of junior, not just having the first score.
The summer before junior is the best time to prep for the tests. The summer after junior should be the time to start working on the applications and essays. In other words, one should have the final scores for picking match, safety, and reach schools by that time.