Biorhythm is an attempt to predict various aspects of a person's life through simple mathematical cycles.(1)
The notion of periodic cycles in human fortunes is ancient; found for instance in natal astrology and in folk beliefs about "lucky days." The 23- and 28-day rhythms used by biorhythmists, however, were first devised in the late 19th century by Wilhelm Fliess, a Berlin physician and patient of Sigmund Freud.(2)
In some ways, biorhythmics resembles chronobiology, the study of biological rhythms. Through medical research, doctors have found that there are periodic biological cycles in a person's lifespan, such as the circadian rhythm (from Latin circa diem; literally, "about a day").(3)
Some biorhythm critics say that biorhythms can be thrown off by such occurrences in the calendar as the beginning of the new year, holidays, or something as simple as the start of the week.
Interpret the readings taking into account the context, the way you feel. People experience sometimes perturbances in their cycles so a reading offset of days is normal.
(1),(2),(3) - http://en.wikipedia.org/wiki/Biorhythm