Full moon days in Hindu calendar are known as Purnima or Pournami. Pournami that falls in the month of Ashadam is called as Ashada Purnima or Ashada Pournami.
South Indian people called it as Pournami, whereas North Indians called it as Purnima.
Apart from festivals, Pournami is a very important and auspicious day in Hindu calendar.
Ashada Purnima is known as Guru Pournami. As per Hindu Scriptures, the day is dedicated to Gurus (Teachers) to pay their respects & express the gratitude to their teachers. The day is also celebrated by Jains and Buddhists.
It is believed that on this day Lord Shiva taught yogic sciences very first time to Saptharishis (seven sages) Agastya, Atri, Bhardwaja, Gautam, Jamadagni, Vashista, and Visvamitra, because of this Lord Shiva is also called as Adi Yogi.