The following graphic explains how you should decide the right time to take your GMAT:
If you are targeting the upcoming MBA session, the month that happens to fall at the intersection of the 2 lines, becomes the best time for you to attempt the test.
If you can afford time, just work forward and take the test when you are best prepared.
Also, notice where I place the “book appointment” marker on the first line. I have tried to explain the ideal time when you should make your GMAT slot booking.