(This post includes excerpt from an old talk by Sri Sri Ravi Shankar)
The day of full moon, Purnima, in the month of Ashadh of the Hindu calendar is traditionally celebrated as Guru Purnima.
Sri Sri Ravi Shankar: Acharya gives knowledge, and a Guru gives diksha. Diksha means the height of awareness and intelligence, and not information. A Guru does not simply stuff you with knowledge, but He kindles life force in you. What happens in a Guru’s presence? You become more alive. Every cell in your body becomes more alive. Guru invokes and awakes the intelligence, not just the intellect. The pinnacle of intellect is intelligence.
Mind is connected with moon, and full moon is a symbol of completion, celebration, or pinnacle. Guru purnima is the day when the disciple wakes up in his/her fullness. And in that fullness, the disciple can’t be but grateful. That gratefulness is not that of you and me, davaita(Duality). But it is of adavaita(Non duality). It is not the river moving to another river, but it is an ocean moving within itself. Both river and ocean move. A river moves from somewhere to somewhere. But where does an ocean move to? It moves within itself. And Guru Purnima symbolizes the fullness of the student, of the disciple. The disciple celebrates in gratitude.