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, worshippig him for His blessings and to gain wisdom. Formerly on this day, gurus who were the traditional teachers, were honoured by their pupils.
In his Laghu-Vaisnava-Tosani, Sri Jiva Goswami has mentioned their family lineage beginning with Sri Sarva, who was a yajur vedi brahmana, Bharadvaja gotriya, from Karnataka. His son was Sri Aniruddha deva who had two sons Sri Rupesvara and Sri Harihar-deva. There was apparently some altercation between the two brothers, who had different mothers (Aniruddha deva had two wives) as a result of which Sri Rupesvara along with his wife and eight horses, came to Paulastya desa, where he was befriended by the ruler of that land, Sri Sekaresvar. Sri Padmanabha deva, the son of Sri Rupesvar was extremely learned in all the Vedas. He came with his family to live at Naihati, on the banks of the Ganga. He had eight daughters and five sons. His five sons, following in the footsteps of their predecessors, were very expert in the study of Vedas. Their names were Purusottama, Jagannatha, Narayana, Murari and Mukundadeva. Mukundadeva's son, Sri Kumara deva, being oppressed by some of the other family members, came to live at Bakla Candradvip. Sri Kumaradeva had many sons, amongst whom Sri Amar (Sanatana), Sri Santosh (Sri Rupa) and Sri Vallabha (Anupama) were great devotees.
"The Caturmasya period begins in the month of Ashadha (June-July) from the day of Ekadasi called Sayana-ekadasi, in the fortnight of the waxing moon. The period ends in the month of Kartika (October-November) on the Ekadasi day known as Utthana-ekadasi, in the fortnight of the waxing moon. This four-month period is known as Caturmasya. Some Vaishnavas also observe it from the full-moon day of Ashadha until the full-moon day of Kartika. That is also a period of four months. This period, calculated by the lunar months, is called Caturmasya, but others also observe Caturmasya according to the solar month from Sravana to Kartika. The whole period, either lunar or solar, takes place during the rainy season. Caturmasya should be observed by all sections of the population. It does not matter whether one is a grihastha or a sannyasi. The observance is obligatory for all asramas. The real purpose behind the vow taken during these four months is to minimize the quantity of sense gratification. This is not very difficult.