Significance
Guru Purnima is dedicated to expressing gratitude to one's spiritual and academic teachers (gurus). It also honours the sage Veda Vyasa, compiler of the Vedas and author of the Mahabharata, and is thus called Vyasa Purnima.
The guru, who dispels the darkness of ignorance, is revered as a bridge to the divine.
Rituals
Disciples visit and offer worship, gifts and gratitude to their gurus. Spiritual seekers begin new studies or sadhana on this day. For Buddhists it marks Buddha's first sermon; it is observed with readings, satsang and charity.
How the date is fixed
Guru Purnima falls on the full-moon day (Purnima) of Ashadha.