Technically, the very best time is when one has completed all of the courses needed for the PCAT plus a few background courses (biochem, genetics, et. Al.). You want to take the test when the info is still fresh. The January date right before the desired admissions scyle is theoretically the best date because it allows the applicant to have a complete primary application right after PharmCAS goes live in June (Note: see caveat below) while not producing results that are too old. There's nothing like being in the front of the line for every school😀. Of course, most people who apply during the second and third years haven't complete all the necessary courses before the January date making the June and August dates more ideal.
Caveat: some schools may prefer a test date that is closer to the time of application review. For example, UNMC is preferring that students take the August or even October PCAT so that the performance is more "current". Just something to think about.....