Guruji and Sharath have always encouraged three days of NOT practicing during menstruation. One reason to take rest during the menstrual period is that the downward and eliminating flow during this time may be counteracted by inversions such as sarvangasana and sirsasana. A second, more subtle, reason is that engaging mula bandha may be more difficult and/or may counteract this downward flow. Without engagement of the bandhas, vigorous practice can be physically unsafe. A third, more general-reason is that excessive activity can lead to an irregular menstrual cycle or the cessation of menstruation (amenorrhea). Menstruation may therefore serve as a convenient time to rest as the body begins the next cycle.
Though taking rest is recommended, it remains an individual choice. As the external and internal practice changes over time, the physical and spiritual importance of taking rest for a particular woman may change as well. If a woman decides to observe ladies’ holiday, she is still very much practicing yoga during this time, as yoga is far more than just asana.