Strategically, you should take the test scheduled around when your personal application deadlines are set. For most schools, you can take the test right up until the day before the app deadline. I suggest that you plan to take the test twice, which means you'll want to take the test for the first time about 40 days before your app deadline at the latest. (The GMAC requires a 31 day waiting period between takings of the GMAT.)
Now, let's talk demand for tests globally.
GMAT demand is pretty flat, meaning there aren't large peaks and valleys in test taking like there are in other standardized tests such as the SAT. That said, most people take the test right before their apps are due, and most apps (for high scoring students) come due in the first quarter of the year, so there is a rush in Dec, Jan, Feb among high-scoring students.
