The word guru is derived from two words, ‘Gu’ and ‘Ru’ . The Sanskrit root “Gu” means darkness or ignorance. “Ru” denotes the remover of that darkness. Therefore one who removes darkness of our ignorance is a Guru. Gurus are believed by many to be the most necessary part of lives. On this day, disciples offer puja (worship) or pay respect to their Guru (Spiritual Guide). It falls on the day of full moon, Purnima, in the month of Ashadh (June–July) of the Shaka Samvat, Indian national calendar and Hindu calendar. 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.[4] 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, around the world.

Hindu Legend

This was the day, when Krishna-Dwaipayana Vyasa – author of the Mahabharata – was born to sage Parashara and a fisherman’s daughter Satyavati, thus this day is also celebrated as Vyasa Purnima. Veda Vyasa, did yeoman service to the cause of Vedic studies by gathering all the Vedic hymns extant during his times, dividing them into four parts based on their use in the sacrificial rites, and teaching them to his four chief disciples – Paila, Vaisampayana, Jaimini and Sumantu. It was this dividing and editing that earned him the honorific “Vyasa” (vyas = to edit, to divide).

“He divided the Veda into four, namely Rig, Yajur, Sama and Atharva. The histories and the Puranas are said to be the fifth Veda.”
– Brahmanda Purana Vyasa Maharshi is the guru of all gurus.All prayers today to ones guru are directly reaching Vyasa.Vyasa maharshi has helped everyone to know vedas and puranas and lead life in a righteous way. Guru means one who removes darkness from ones life.
Buddhist History

The Buddha went from Bodhgaya to Sarnath about 5 weeks after his enlightenment. Before Gautama (the Buddha-to-be) attained enlightenment, he gave up his austere penances and his friends, the Pañcavaggiya monks, left him and went to Isipatana (Sarnath).

After attaining Enlightenment the Buddha, leaving Uruvela, travelled to the Isipatana to join and teach them. He went to them because, using his spiritual powers, he had seen that his five former companions would be able to understand Dharma quickly. While travelling to Sarnath, Gautama Buddha had to cross the Ganges. Having no money with which to pay the ferryman, he crossed the Ganges through the air. When King Bimbisara heard of this, he abolished the toll for ascetics. When Gautama Buddha found his five former companions, he taught them, they understood and as a result they also became enlightened. At that time the Sangha, the community of the enlightened ones, was founded. The sermon Buddha gave to the five monks was his first sermon, called the Dhammacakkappavattana Sutta. It was given on the full-moon day of Asadha. Buddha subsequently also spent his first rainy season i.e. Varsha vassa at Sarnath at the Mulagandhakuti. The Sangha had grown to 60 in number (after Yasa and his friends had become monks), and Buddha sent them out in all directions to travel alone and teach the Dharma. All 60 monks were Arahants.
Observances by Buddhists and Hindus

Buddhists observe on this day uposatha i.e. to observe eight precepts. Vipassana meditators practice meditation on this day under the guidance of their teachers. Rainy season i.e. varsha vassa also starts with this day. During the rainy season lasting for three lunar months from July to October. During this time Buddhist monks remain in a single place, generally in their temples. In some monasteries, monks dedicate the Vassa to intensive meditation. During Vassa, many Buddhist lay people reinvigorate their spiritual training and adopt more ascetic practices, such as giving up meat, alcohol, or smoking.
A sanyasi performing Vyasa puja traditionally held on Guru Purnima day, as a part of Chaturmas rituals

The Hindu spiritual Gurus are revered on this day by remembering their life and teachings. Vyasa Puja is held at various temples, where floral offerings and symbolic gifts are given away in his honour and that of the cosmic satguru. The festivities are usually followed by feast for the disciples, shishya, where the prasad and charnamrita literally nectar of the feet, the symbolic wash of Guru’s feet, which represents his grace, kripa is distributed.

As a day of remembrance towards all gurus, through whom God grants the grace of knowledge (Jnana) to the disciples, special recitations of the Hindu scriptures especially, the Guru Gita, a 216 verse ode to Guru, authored by the sage, Vyasa himself, are held all day; apart from singing of bhajans, hymns and organising of special kirtan session and havan at many places, where devotees from all over gather at the ashrams, matha or place where the seat of Guru, Guru Gaddi exists. This day also sees the ritual of padapuja, the worships of Guru’s sandals, which represent his holy feet and is seen a way of rededicating to all that a Guru stands for. Disciples also recommit themselves on this day, towards following their teacher’s guidance and teachings, for the coming year.

A mantra that is particularly used on this day is “Gurur Brahma, Gurur Vishnu, Gurur Devo Maheshwara, Guru Sakshat Parabrahmah Tasmai Shree Guru Veh Namah”.

This day is also seen as an occasion when fellow devotees, Guru Bhai (disciple-brother), express their solidarity to one another in their spiritual journey.
Jainism

According to Jain traditions, it was on this day, falling at the beginning of chaturmas, the four month rainy season retreat, Mahavira, the 24th Tirthankara, after attaining Kaivalya, made Indrabhuti Gautam, later known as Gautam Swami, a Ganadhara, his first disciple, thus becoming a Guru himself, therefore it is observed in Jainism as Guru Purnima, and is marked special veneration to one’s Gurus and teachers.