In the Hindu scriptures guru who kindles the urge for self-realization has been praised by the verse: "Gurur Brahma Gurur Vishnu Gurur Devo Mahesvarah; Gurur Saakshat Para Brahma Tasmai Shree Gurave Namah" meaning that my salutations are to the noble teacher who is Brahma, Vishnu and Parmeshwara or the supreme soul.
A true guru is supposed to be a person of great scholarship and free from lust, anger, greed, infatuation, pride and envy, In other words, guru has to be living example and illustration both to clarify and to inspire the students to live a profound life.
Most Hindus regard sage Veda Vyasa as the universal guru who compiled four Vedas, and later wrote Upanishads and 18 puranas to explain and clarify the principles mentioned in the Vedas. Not only that, he also wrote the epic Mahabharata and later authored 18 upa-puranas revered by the Hindus.
Veda Vyasa was born on Ashadha Purnima, full moon day in the lunar month of Ashadha. It is also the day he completed 'brahms sutras' considered being the most important works in Vedanta philosophy. The Hindus celebrate guru purnima on Veda Vyasa's birth day as the day of transmission of experiential and existential from the enlightened teacher to his or her student.
Thus, for the Hindus, Guru Purnima is the day to honour the Self-realized masters with humble reverence, by getting initiated to path of sincere inquiry into the truth of life.