“The hardest training that takes place in a Zen monastery during the year is known as the rohatsu dai sesshin, an intensive period of Zazen lasting from the first of December to the morning of the eighth, that commemorates the enlightenment of the historical Buddha, Sakyamuni. The word "sesshin" means to concentrate one's mind. This is accomplished through day and night practice of zazen and private interviews with the roshi during the week-long period.” ZenSesshin Book:The Zen Life Source: The Zen Life