When setting up a Study Schedule with students, I like to leave an open slot the week before the exam, so we can decide as we get closer to your GMAT test whether you should do another “last minute” CAT. Why? Because in my experience there is no hard and fast rule. Some of my students just need a break the two days before the exam to mentally “rest up,” while others seem to enjoy re-taking a GMATPrep 3 or 4 two days before just to move the flow of “official” questions front-and-center.
So what should YOU do? Ultimately, see how you feel the week of, and make your own decision. If you’re feeling burnt out or worried it might make your anxious, definitely avoid it. There’s nothing you’re really going to be able to cram in the 48 hours before the test that’s going to make that great a difference to your score. So the only benefit to a CAT that close to your exam is if you think of it like a prolonged runner’s stretch.
If you were to take a practice test right before, then I’d do it ONLY to keep your pacing “in shape” and your brain in the GMAT-space, and I’d still NOT suggest doing it the day/night before your exam but rather two days before. If you do decide this is the way to go, I’d suggest a GMATPrep re-take and NOT any new practice tests (such as one from Veritas or MGMAT). There’s no point in taking a brand new practice test if you can’t properly examine the errors, and whatever score that is churned out by the private test prep company could likely unnerve your or undermine your confidence.
If you can take the CAT with a sense of “play” and “muscle-flexing” then go for it, if it seems like a horrible stress nightmare that will lead to sleepless nights, then definitely avoid.