Why seven days or a multiple of seven days? Our mental states are influenced by our physical states, which in turn are influenced by the cosmos. Nature itself seems to take seven days as a definite period. This concept of seven days is very ancient. It may come from the observation of celestial bodies. In the Bible, God created the world in seven days. In India the seven days were related to the seven planets. In ancient China this period was called "one come-and-go."

Our body is a small universe, a microcosm, and it tends to reflect the great universe, the macrocosm. Our body and mind seem to demonstrate a cycle of seven days. Thus we take seven days as the optimum period of practice. So we can go on the first seven days, the second seven days, and the third seven days to help us to get onto a smooth and diligent path of practice. In Japan it is called sesshin, meaning "uniting or transmitting, the mind." This can be interpreted on one hand, as the roshi, or teacher, taking away the student's vexed mind and enabling him to achieve enlightenment. On the other hand, the student takes the mind from the roshi.