Buddhists believe that, on this date, the Buddha's spirit was freed when he attained enlightenment under a sacred Bodhi tree in Buddh Gaya, India.
Buddha had supposedly vowed to never arise from beneath the tree unless and until he had discovered the truth at the core of existence. Tradition has it that it takes him 49 days of meditation to achieve this, and it is at this point that Siddhartha Gautama becomes known as the Buddha, or "Awakened One."
Buddhism teaches that the Buddha has acquired complete insight into the cause of the suffering all humans experience, as well as how humans can alleviate it. Learning all of this is at the heart of Buddhist teaching and practice.
Anyone who does master it all achieves Nirvana, a state which is free from suffering, greed, ignorance, hatred, etc. It is also supposed to be the elimination of all personal identity and boundaries.