Q has been asked and answered before.
The testing center is NOT only for the GMAT but also for other medical exams that docs and all take. So some days are blocked off exclusively for them. However, if the docs dont book a slot then it opens up for the GMAT.
Also, the center is open 6days a week with 1rest day in between for them(come on ya'll they deserve a break don't they

) however, once it reaches peak season, there are slots on all days since demand is high blah blah blah.
Honestly, you'll should stop worrying about whether its advantageous to take it on a particular day/center/time and the likes. Its freakin adapative and the computer doesn't care where you take the test. Don't try to game the system you'll get screwed. Take the test when you are ready. Period.
You can pre-pone/post-pone the exam its all up to you. However remember that each time you do that it costs you $50. I'm not sure about pre-poning i think they've taken off that option but you can always check.