A teacher is the 'light' who removes the darkness of ignorance from the life of his disciple. He lights the path, corrects mistakes and leads his students on to righteousness. Guru Purnima is a day during which one can thank one's past and present teachers and get their blessings for a bright future ahead.
Falling on the fifteenth day of the bright fortnight, during the Shukla Paksh in the month of Ashadha, Guru Purnima is celebrated as a day for paying gratitude to one's Guru, basically the spiritual guide as per Hinduism, Buddhism and Jainism.
Historically, it is the birth anniversary of Guru Vedvyas, believed to be the first Guru who is also credited to have authored the Mahabharata and the Vedas and Puranas. Having given such valuable sources of learning to the world, Hindus offer their prayers to Guru Vedvyas.
Another story says that Lord Shiva had given the secret of Yoga and higher spiritual consciousness to his seven disciples, collectively known as the Saptarishis on this day. Therefore, Lord Shiva is also called Adi Guru - which is a Sanskrit term translating to the first Guru, or the first teacher.
In Buddhism, this day is celebrated as the day when Lord Buddha had served his first Sermon at Sarnath. Thus, Lord Buddha is offered prayers on this day.
In Jainism the day is celebrated as Trilok Guhu Purnima, a day when Lord Mahavira, the 24th Tirthankara had made Indrabhuti Gautam, his first disciple.
Irrespective of who you are offering the prayers to, the day holds deep spiritual significance, and everyone must honour one's teacher on this day.
The procedure to perform puja for Lord Shiva or Guru Vedvyas is as follows:
On the Purnima day, wake up before the sunrise, and take bath in the Brahma Muhurta.
Make a Vyas peeth, on a white cloth, at the place of worship. If you do not know how to make a Vyas Peeth, it is better to take the help of a priest.
Sprinkle Gangajal, light the lamp and incense stick.
Chant the mantra 'Gurur Brahma Gurur Vishnu Gurur Devo Maheshvarah Guru Sakshat Par Brahma Tasmaye Shri Guruve Namah'.
Offer all the sacred things including dhoop-deep, flowers and sweets before the image of Guru.
Throw rice (akshata) in all the directions after this.
Invoke Lord Shiva, Lord Buddha, Lord Mahavira, Mahatma Vedvyas and other spiritual Gurus that you want to offer your prayers to.
Then you can offer the Guru Prasad to a priest or a Guru whom you have invited to offer the Guru Purnima feast to.
Thereafter, honour your teacher with a gift as Dakshina.
At the end, do not forget to distribute prasad among the family members and to make some donations to the needy.