Let's take nature as an example: there are four biological cycles: fall, winter, spring and summer. It is repeatable process that never stops. People also have their personal cycles such as emotional, physical and intellectual called biorhythms. According to the theory of Biorhythms, each of these rhythms goes through three different phases; high, low and critical.
Generally speaking, the more positive a cycle is at any given point in time, the better one is able to interact in that arena. For example, high emotional level tends to mean that a person is more stable, is better able to make relationship decisions, and so on. This is not to say that when the cycle is in the negative range that the person is not doing well in that arena, rather, it means that it is harder to do well. Plan exams when your Intellectual cycle is high.
According to the theory, when certain points on the cycles are reached a person may enjoy special strength or suffer special weakness. "Switch point days," when cycles cross the zero line on the ascent or descent, are "critical" days. Performance on critical days is supposedly very poor. It has even been predicted that people are especially accident prone on critical days. In fact, studies have show then there is a 30-60% greater risk of an accident during those few critical days that a person experiences every month. It can pay to be more cautious on these days.
In any case, according to the theory of biorhythms, critical days are days you want to know about in advance so you can prepare for them. For example, if you are scheduled to take a test that will measure your thinking ability, make sure you do not take the test on a day when your intellectual cycle is at a critical or a low point. Of course, to do well one must also get a good night's sleep, be generally healthy, eat properly, and study, but those preparations will do you no good if your intellectual cycle is not in the right spot. On the other hand, if you are a long distance runner, try to pick your next race date so that you are at a peak on your physical cycle. Of course, you must train properly, eat well, get sufficient rest, be healthy, etc., but these will not suffice if your physical cycle is at the wrong point.