Definition of Biorhythm from Wikipedia:

"The theory of biorhythms claims that one's life is affected by rhythmic biological cycles, and seeks to make predictions regarding these cycles and the personal ease of carrying out tasks related to the cycles. These inherent rhythms are said to control or initiate various biological processes and are classically composed of three cyclic rhythms that are said to govern human behavior and demonstrate innate periodicity in natural physiological change: the physical, the emotional, and the intellectual (or mental) cycles. Others claim there are additional rhythms, some of which may be combinations of the three primary cycles. Some proponents think that biorhythms may be potentially related to bioelectricity and its interactions in the body.

Basic rhythms follow certain facets of physiological cycles, though they may include others, and the details may vary depending on the source. The three classical cycles of biorhythms are endogenous infradian rhythms. The theory's basis lies in physiological and emotional cycles. They are often represented graphically as either symmetric or asymmetric waveforms, though most theories rely on symmetric forms. The most commonly used form is the sinusoidal waveform, which is thought to be a plausible representation of a bioelectric activity cycle. Due to this sinusoidal nature, the cyclical flow of bioelectric activity undergoes periodic reverses in direction. Each cycle oscillates between a positive phase [0%..100%] and a negative phase [-100%..0%], during which bioelectric activity strengthens and weakens. The waveforms start, in most theories, at the neutral baseline (0%) at the time of birth of each individual. Each day that the waveform again crosses this baseline is dubbed a critical day, which means that tasks in the domain of the cycle are far more erratic than on other non-critical days. The purpose of mapping the biorhythmic cycles is to enable the calculation of critical days for performing or avoiding various activities."

Reading your biorhythm chart

Your biorhythm chart has the calendar dates printed on the bottom and the cycles running above them.  There is an axis running down the middle that represents the critical point of each cycle and there are guide lines to help mark each day. The cycles themselves are represented by the three colored curves:  physiological, emotional, and intellectual. On day 1 each cycle falls exactly on the axis. On the mid-cycle day it once again crosses the axis.

For the odd numbered cycles (physical and emotional) the character representing that cycle does not fall on the axis at mid-cycle.  That's because they cross the axis at 12 1/2 and 17 1/2 days.  Therefore, critical days at mid cycle for the physical and intellectual cycle are when the cycle's character is one space above the axis one day and one space below it the next.

More about cycles that rule our lives: Kondratiev wave, “Early-warning signals for critical transitions.”