GURU POORNIMA:

This was the day, when Krishna-Dwaipayana Vyasa – author of the Mahabharata – was born to sage Parashara and a fisherman's daughter Satyavati, thus this day is also celebrated as Vyasa Purnima.^[4]Veda Vyasa, did yeoman service to the cause of Vedic studies by gathering all the Vedic hymns
extant during his times, dividing them into four parts based on their use in the sacrificial rites, and teaching them to his four chief disciples – Paila, Vaisampayana, Jaimini and Sumantu. It was this dividing and editing that earned him the honorific "Vyasa" (vyas = to edit, to divide). "He divided the Veda into four, namely Rig, Yajur, Sama and Atharva. The histories and the Puranas are said to be the fifth Veda."
- Brahmanda Purana
Even it is celebrated as Sai Baba's puja day or day when he appeared in shiridi ..it is celebrated grandly all over india in dattakhetryas and sai temples especially in shiridi