Biorhythms!
What kind of day is it?
That is exactly the sort of question that the study of biorhythms is supposed to answer. Biorhythms are the body's cycles that fluctuate on a regular and predictable schedule. The biorhythm chart will measure four distinct fluctuations, physical, intellectual, emotional and intuitional based on the daily changes of each.
Each fluctuates at its own speed, but all start off at zero on the day you were born. Therefore simply by calculating how many days you have been alive, the biorhythm chart can map out your readings for any given day.
Knowing the high and low points of the rhythms and the "critical periods" where they cross over from positive (active) to negative (inactive) and vice versa just might give you an edge on what kind of activities to plan and avoid for certain days. Lots of people swear it works, while many others scoff at it.