The full-moon day (purnima) in July is the auspicious (Poornima) day when Guru Poornima is celebrated by worshipping our Guru, the spiritual master. A qualified spiritual master is essential to learn self-realization. Bhagavadgita (Ch.4.34) asks one to learn the truth through a Guru or spiritual master, inquire from him submissively and render service unto him. The self-realized soul imparts knowledge as he has seen the truth. A real Guru imparts gynana (true knowledge) and teaches how to raise one's consciousness beyond material physical boundaries.
In essence there is only one Guru. There cannot be two Gurus - inner Guru and external Guru. Guru is one who treads along the path of Truth. He is truth himself.
Gurur-Brahma Gurur-Vishnu Gurur Devo Maheswara; Guru Sakshat Param Brahma Thasmai Sri Gurave Namaha. (Guru is Brahma, Guru is Vishnu, Guru is Maheswara. Guru is verily the supreme Brahman. So, salutations to the Guru.)
You are the embodiment of Divine Trinity - Brahma, Vishnu, Maheswara. Hence, Guru is not separate from you. You are everything. Let your concentration not waver. Let it be steadily fixed on your goal. Be he a student, a celibate, a householder or a renunciant, the goal is one and same for all. Contemplate on the principle of So-ham that your inner voice teaches. Say Aham Brahmasmi (I am Brahma). There is nothing wrong in this statement. Some people have a mistaken notion that to say so, is a sign of egoism. In fact, that is not ego at all. It is your right. To think that you are a mere human being amounts to demeaning yourself. You are the embodiments of God. Develop such faith and become God. There cannot be any bad qualities in God. So, you should lead a pure, steady and selfless life. See good, speak good and do good. You should be an ideal to everybody. No benefit accrues from worship if you do not purify your heart.
This mantra celebrates and venerates the guru inside us that leads us to light. Chanting this mantra on the day of Guru Purnima is considered especially auspicious.