So there is something in psychology called discounting. Basically, if you bring a group of people in and ask them how their life is going overall, they'll tell you it's worse if you brought them in on a rainy day than if you met them on a sunny day. However, this effect goes away when you call attention to whats causing the rating change (e.g. "please do not factor today's weather into your evaluation").
Basically, people have trouble distinguishing between how they feel right this second, and how they've felt over time. UNLESS you make yourself aware of those factors, allowing you to discount them. So if you wake up the morning of your GMAT and spill your orange juice all over your best loafers, or the weather sucks, or your dog leaves you, or your wife pees on your prep notes, stop for a second and realize that these events have nothing whatsoever to do with how you've prepared, and that they in no way effect your abilities. If you don't discount them, they may drag you down.