Critical day
A critical day is the day on which a biorhythm cycle crosses the zero line and changes phase: from high to low or from low to high. The classic method treats it as the least stable point of the cycle, where that area is most exposed, and calls for attention rather than fear. Every cycle has two per turn.
On the chart it is the point where the curve cuts the centre line. There are two per cycle: one when the cycle restarts and the curve rises, one halfway through, when it falls. The method tells the ascending critical from the descending one, and calls the day before the crossing precritical.
With three cycles of 23, 28 and 33 days, critical days come round roughly every eleven, fourteen and sixteen days. They rarely all land together: when two coincide it is called a double critical day.
For someone born on 15 June 1990, on 2 September 2026 the emotional cycle is on day 13 of 28: the next day, day 14, it crosses zero into the low phase, so today is a precritical day and tomorrow a descending critical day.