Ash,
I had a similar question when I was preparing. Over the course of time, I just checked the Prometric site near me in the Washington, DC area and looked at how many slots were available and how quickly they were filling up. My observation for my area was that the prime slots went quickly, but others during weekdays were abundant enough that I was pretty certain I could get one compatible with my timing. I ended up booking mine about 2 weeks in advance and it worked out fine. I was also helped by the fact that it was not a problem for me to take the day off for the test.
Of course, if you're pushing up against the PMBOK Version 6 deadline, I don't know what you'll find. If a large number of people are rushing to get in while version 5 is still in effect, it could be more difficult. Bear in mind that you can schedule a date but then change it, as long as you change far enough in advance. I don't recall off hand how far in advance you need to be in order to not incur a $$ penalty, but you can look that up on the PMI site.
Good luck!
Tracy