There is a Shloka that states -
Agyan timirandhasya gyananjan shalakaya
chakshurunmilitam yen tasmai shri gurave namah
Gurur Brahma, Gurur Vishnuhu, Guru Devo Maheswaraha
Guru Saakshaat Parabhrahma, Tasmai Sri Gurve Namah
Traditionally the festival is celebrated by Buddhists in the honor the lord Buddha who gave His first sermon on this day at Sarnath, Uttar Pradesh, India. While Hindus celebrate it in the honour of the great sage Vyasa, who is seen as one of the greatest gurus in ancient Hindu traditions, and a symbol of the Guru-shishya tradition.
Vyasa was not only believed to have been born on this day, but also to have started writing the Brahma Sutras on ashadha sudha padyami which ends on this day, hence their recitations as a dedication to him, are organised on this day, which is also known as Vyasa Purnima.
The festival is common to all spiritual traditions in Hinduism, where it is dedicated to the expression of gratitude towards the teacher by his/her disciple.
Hindu ascetics and wandering monks (sanyasis), observe this day by offering puja to the Guru, during the Chaturmas, a four month period during the rainy season, when they choose seclusion, and halt at one selected place; some also give discourses to the local public. Students of the Indian classical music, which also follows the Guru shishya parampara, celebrate this festival.
SOURCE: Wiki and Hindu blog.
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