Bodhi day (also know as Rohatsu) is always on the 8th day of the 12 lunar month of the Chinese calendar or 8th December and is celebrated by Mahayana Buddhists around the world. It commemorates the day when the Buddha, (Siddhartha Gautama) received enlightenment in 596BC under the Bodhi Tree. 

Potted History of the Buddha

Siddhartha Gautama, was born to a wealthy family in Nepal. He was believed to be a Prince. He lived a very sheltered life and really wanted to explore the world and travel. In his travels he saw much misery and suffering, which affected him deeply. At the age of 29 his left his comfortable home to travel and seek the "meaning of life". (Something I think many of us do).

He studied with many teachers as he travelled widely.. He explored different countries and the religions of that time but he soon realised that this was not how to find the answer to that all important question. He decided to meditate in the shade of a tree (which happened to be a Bodhi) until he had his answer. He fasted and meditated for a week and on the 8th day (which also happened to be his 35th birthday) he came to the realisations which lead to The Four Noble Truths and the Eightfold Path which are the governing principals of all forms of Buddhism.

So this day can be celebrated by everyone as a day of enlightenment. For Buddhists it tends to be a day of meditation and remembrance very much like many holy days of all religions. Lights and candles are lite to symbolise enlightenment. Often small trees are brought inside and decorated much like a Christmas Tree. Sometimes Bodhi trees are decorated with 3 shiny ornaments to represent the Three Jewels, the Buddha, the Dharma (the path of righteousness by living one's life according to the codes of conduct as described by your religion - in terms of Buddhism this is protection from suffering by following the teachings of Buddha) and the Sangha (Community).