Here's how I break down when people should take the test:
June 23: If you have just completed your prerequisites this past spring so that the information is still fresh in your mind. Most colleges get out in early May, so two months is plenty of time to review stuff. This date is ideal for those applying to ED and/or rolling admission schools. People who have finished their second year and want to get in during third year should take the test on this date.
August 25: This is the second-most ideal date for those people who haven't had the prereqs in a while, like people who have finished their third, fourth, or even graduate years. Though June 23 will work for these people, some people may need an extra two months to review. In addition, people who have just finished their first year probably should take this test by using the entire summer to study the content from prereqs that won't be taken until their second year.
October 20: Realistically, this is the last date many pharmacy schools will accept if you want to get into pharmacy the next school year after this test. Ideally, this date should be for people who didn't do so well on the previous PCATs. This is not a good date for rolling admissions, though it isn't impossible if one gets his/her application materials in prudently.
Also, this is the designated procrastinator's date for taking the exam.
January 19: This is really ideal for people who want to apply to ED and rolling admission schools who plan not to be in pharmacy school the following school year. That is, you take this test, do another year of undergrad/graduate/something during the next school year while applying during the summer, and get in the year after that. I suppose that people applying to schools with a March 1st deadline may have a chance, but this is a really bad date in terms of timing. Don't take this exam if you want to be in pharmacy school the following school year. Regardless, there isn't much time for studying with Christmas and New Year's, but this allows your application to be processed and finished the soonest.
To sum up:
January: Ideal time for ED/rolling applicants who want their application processed as early as possible
June: People just finishing their prereqs + ED/rolling admissions
August: People who haven't taken their prereqs in a while and need a review + people who haven't taken all their prereqs yet + people applying primarily to regular admission schools
October: People who didn't do to well on their first PCAT + procrastinators
Of course, these are ideal dates and we all know that people rarely follow the ideal dates.