It is not that a single day be use as a mark of respect towards our elders and teachers but Guru Purnima defines the importance of respect a pupil must have for his or her teacher. The mantra below defines the significance more clearly.
Gurur Brahma Gurur Vishnu Guru Devo Maheshwarah
Gurur Sakshat Parabrahma Tasmai Shree Gurave Namah
The preceptor is verily the creator, preserver and the destroyer of the worlds. In this regard, the Guru is in fact equal to Brahma, Vishnu and Shiva, the holy triad of Hinduism. Over and above, the preceptor is verily the Supreme Soul to whom I pay my respect and worship.
This is the day to revere, respect and pay obeisance to the Guru or the preceptor who removes the darkness of ignorance and lights the lustrous lamp of wisdom in the hearts of pupils. Also called as Vyas Purnima, this day is especially dedicated to worship Maharishi Veda Vyas, the great Guru and sage who compiled all the four Vedas, eighteen Puranas, the Mahabharata epic and Srimad Bhagavatam besides several other sacred scriptures of Hinduism.
For Buddhist, the day is auspicious because on this day Lord Buddha gave his first sermon at Sarnath, Uttar Pradesh. The festival is also believed to mark the onset of monsoons and therefore a source of celebration for farmers.