'Dharma' can be translated as truth and is the term used for the teachings for the path to enlightenment.
Dharma Day is celebrated in Theravada countries on the full moon day of July. It marks the beginning of the Buddha's teaching. Soon after the Buddha's enlightenment, he went to find his disciples and tell them about his experience. The Buddha's first sermon is believed to have taken place in the Deer Park at Sarnath. This event is usually seen as the beginning of Buddhism and is what Dharma Day celebrates.
The first teaching to the Buddha's original five disciples is known as The First Turning of the Wheel of the Dharma. In this first sermon the Buddha taught the Four Noble Truths.
The time around what has now become Dharma Day marks the beginning of the rainy season, called the 'vassa'. Traditionally, in early Buddhism, at this point, the Buddha and his monks and nuns would suspend their nomadic lifestyle for three months. They would shelter together until the rainy season was over, and use this time as a period of further meditation and reflection. At the end of this time, they would resume their travelling, passing on the Buddha's teachings to those who were interested. Modern monks still spend the 'rainy season' in a monastery rather than travel.
Dharma Day is now seen as a chance to express gratitude that the Buddha and other enlightened teachers have shared their knowledge with others. Dharma Day is usually celebrated with readings from the Buddhist scriptures, and is an opportunity to reflect deeply on their content.
Dharma Day is important to Buddhists today as it reminds them of the Buddha's teachings and what they can learn from his enlightenment and apply to their own lives.