Bodhi Day commemorates the enlightenment (bodhi) of Siddhartha Gautama, the night and dawn on which, after years of seeking and a final period of meditation beneath the tree later called the Bodhi tree at Bodh Gaya, he saw through to the nature of suffering and its ending and became the Buddha, the "Awakened One". It is the central event of Buddhism, the moment from which the whole tradition flows, and its commemoration is accordingly one of the most significant days for those schools that keep it separately.
The observance belongs above all to the Mahayana traditions of East Asia. In Japan, where it is called Rohatsu, it is fixed to 8 December (the eighth day of the twelfth month), and in the Zen schools it is preceded by an intensive week-long meditation retreat, the rohatsu sesshin, in which practitioners sit through the nights in emulation of the Buddha's own resolve not to rise until awakened. Elsewhere in the Mahayana world it follows the traditional lunar date. The Theravada traditions of South and Southeast Asia do not keep a separate Bodhi Day, commemorating the enlightenment instead as one of the three great events, birth, enlightenment and passing, gathered together on Vesak. Where it is kept, Bodhi Day is observed quietly, with meditation, study of the teaching, chanting and reflection rather than festivity, in keeping with the inward character of the awakening it recalls.