Rohatsu is Japanese for "eighth day of the twelfth month." December 8 has come to be the day Japanese Buddhists observe the enlightenment of the historical Buddha.
In Japanese Zen monasteries, Rohatsu is the last day of a week-long sesshin. A sesshin is an intensive meditation retreat in which all of one's waking time is dedicated to meditation. Even when not in the meditation hall, participants endeavor to maintain meditation focus at all times -- eating, washing, doing chores. Silence is maintained unless speaking is absolutely necessary.
In a Rohatsu Sesshin, it is traditional for each evening's meditation period to be longer than the previous evening's. On the last night, those with enough stamina sit in meditation through the night.
The Buddha's enlightenment is observed at different times in other parts of Asia. For example, Theravada Buddhists of southeast Asia commemorate the Buddha's birth, enlightenment and passing into Nirvana at death on the same day, called Vesak Puja, which is usually in May. Tibetan Buddhists also observe these three events in the life of the Buddha at the same time, during Saga Dawa Duchen, which usually is in June.