Guru Purnima is to commemorate all masters and gurus for their role in guiding souls to realize God.
Wikipedia, the free encyclopedia defines Guru Purnima as a Hindu festival dedicated to spiritual and academic teachers ... marked by ritualistic respect to the Guru, Guru Puja. The Guru Principle is a thousand times more active on the day of Gurupournima than on any other day ... 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. Hindus celebrate it in 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 .... this day, which is also known as Vyasa Purnima.
The Isha Foundation states in its blog at http://www.ishafoundation.org/Isha-Celebrations/guru-poornima.isa, "This sacred day marks the very first transmission of the yogic sciences from Shiva – the Adiyogi or First Yogi – to the Saptarishis, the seven celebrated sages"
A beautiful revelation by Ram Dass on the coming of the guru and his role is carried at http://www.ramdass.org/the-consciousness-of-saints/
Finally to sum it all up, Ram Dass quotes from Dada Mukerjee's book THE NEAR AND THE DEAR available at http://www.ramdass.org/work-saints/
The Work of Saints
Saints are one in their work as divine channels. They link us up with the source from which all bliss flows: love. They bake the unbaked pots and make them fit to receive the divine bliss. They illuminate the path by removing all the darkness that holds us back. Their work does not end with the energizing or enlightening of some select few here and there. They also serve as the unflickering flame helping to light other candles. What bliss and joy Hanuman brought to his devotee Tulsidas! And what bliss and joy Tulsidas himself has scattered to millions of other devotees, helping to rekindle their lamps, although he himself parted with his body long, long ago. This has been the case with many great saints and their disciples, from Guru Maharaj to Shyama Charan, from Ramkrishna to Vivekananda. Through them and their own disciples they live in the memory of many old people. The work of the saints goes on, whether we know it or not.
- Excerpt from The Near and the Dear by Dada Mukerjee