The appearance anniversary of Srila Vyasadeva, the compiler of the Vedic literature.
This event really is directed towards Srila Vyasadeva, thus in some sampradayas they refer to Guru Purnima as Vyasa Purnima. Traditionally it is the day when the guru is worshipped.
In the book Festivals, Fairs and Fasts of India (Shakti M Gupta. 1991. Clarion books. page 88-89.) It says: Guru Purnima "……is observed on the full moon day in the month of Ashadha in honour of the sage Vyasa, by keeping a fast, wor
