If you apply early decision, I would suggest the August PCAT. However, if you want more than one shot at the PCAT for scores that will be considered for ED, you should consider June and August.
I took the PCAT in august this year, and applied ED for fall of 08. You must have the application submitted to PharmCAS by the ED deadline, if they do not have your transcripts and LOR by then, they will not mail your application to the college. My LORs were in by the end of July, and my last transcript trickled in mid-August. I submitted on Aug 23rd and it was mailed Sept 4 for a Aug 31 deadline. I was missing one transcript for my summer classes (I took 13 credits) and I called PharmCAS. They told me that I could wait for the other transcript to send it, or I could just state them as still being in progress, and they would send it right away. They advised me that if I were to wait, I may miss the opportunity.
The sooner you get your applications in, the better, as there might not be enough interview slots, or if your application isn't mailed out from PharmCAS, you could lose out.
Early Decision applicants were guaranteed a notification of acceptance, rejection or deferral by October 20th. Which gives you a pretty good amount of time to apply elsewhere, but some of the windows are very small depending on where you are considering. Deferral of course opens your opportunity to apply elsewhere.