Today, more people use the word guru in their conversations and if you listen, they mean someone who is super good at a subject. It is actually a misuse of the word because the word has its origins in the original language Sanskrit where gu means darkness and ru means one who removes darkness. Here, darkness is symbolic of ignorance and guru is a person who removes ignorance and adds knowledge. This applies to both spiritual and academic teachers, for Hindus, spiritual salvation can be achieved through, among other things, a guide. Therefore, a teacher or guru has a solemn position in society. In music and dance, one is often judged by which teacher one has had. Purnima means full moon and the day marks the first peak of the lunar cycle after the solar cycle. The meaning of the celebration becomes 1000 times more active on the day of the full moon than on any other day.
When it comes to academic teachers, something that all people have throughout their lives, India is the only country I know of where such respect for teachers is practiced. Indian academics celebrate this day by thanking their teachers as well as remembering past teachers and scholars. In the yoga tradition, the day is celebrated as the occasion when Shiva became the first guru, when he began the transmission of yoga to the saptarishis (monks/sages). Students of Indian classical music and classical Indian dance, who also follow the Guru shishya Parampara (transmission of teachings symbolized by the lighting of lamps), celebrate this sacred festival all over the world. Gurupurnima is the day when Krishna-Dwaipayana Vyasa - author of the Mahabharata - was born and the day is celebrated as Vyasa Purnima. He is called Veda Vyasa, who was the mastermind behind Vedic studies and managed to divide them into four parts based on their use in sacrificial rituals, and taught them to his four foremost disciples – (Paila, Vaisampayana, Jaimini and Sumantu). It was this division and editing that earned him the honorary title "Vyasa" (vyas = to edit, to divide). "He divided the sacred Veda (knowledge/teachings) into four, namely Rig, Yajur, Sama and Atharva. The Histories and Puranas are said to be the fifth Veda." All Hindus are indebted to this ancient saint who edited the four Vedas, wrote the 18 Puranas, the Mahabharata and the Srimad Bhagavatam.
Here is a picture of Veda Vyasa, source: Wikipedia