Dharma Day or Asala Puja celebrates the first discourse of the historical Buddha’s first discourse following his spiritual awakening.
It is one of the most important dates in the Buddhist calendar as it marks the day the religion was established. It is usually celebrated with readings from the Buddhist scriptures, and is an opportunity to reflect deeply on their content. Also known as Asalha Puja, Dharma Day falls during the eighth month of the traditional Indian calendar. (source)