According to both Hindu and Buddhist traditions, Guru Puja or Guru Purnima is celebrated on the full moon day of the lunar month of Ashadha, which corresponds to June-July of the Roman calendar.
To this day Guru Purnima is one of the major celebrations in Shirdi. Shri H.V. Sathe, one of the early and close devotees of Shirdi Sai Baba, recounts, "Baba's arati and puja on ordinary days were started, but for some time there was no celebration of Guru Purnima by special worship of Baba on that day. Baba himself sent for my father-in-law (Shri Dada Kelkar) and said, ‘Do you not know that this is Guru Purnima? Bring your worship material and do your Guru Puja.’ Then we had Guru Puja on that day and this practice of celebrating Guru Purnima by all the devotees worshipping Baba then begun, has continued to date." [From Devotees’ Experiences of Shirdi Sai Baba, by B.V.Narasimhaswami]
The above account of Sathe has given way to the general belief among Sai devotees that as Baba “himself” sent for Dada Kelkar, reminded him of the Guru Purnima and asked him to render puja, the custom of celebrating Guru Purnima at Shirdi was founded by Baba himself. But a letter published in the “Shri Sai Leela” (monthly magazine) throws some light on the actual antecedents of the event. The letter was written by Shri Tatyasaheb Noolkar to Shri Nanasaheb Chandorkar. Unfortunately the date of the letter is not mentioned.
The letter reads:
"On Saturday just as I got up in bed, I remembered it was Guru Puja day and made preparations. After bhiksha [begging rounds of Sai Baba] we were permitted to perform Guru Puja. Baba allowed us to do all upachar [particulars and points of worship] quietly. He returned all dakshina that was given at the time of puja. Then He sent word to Radhakrishna and Dadabhat [Dada Kelkar, father-in-law of Rao Bahadur Sathe] that puja and arati was being done and they should come soon. So Radhakrishna sent her puja [materials] and Dada came with his puja articles."
It is evident from the above letter that some devotees who knew that it was Guru Purnima implored Baba to allow them to do Guru Puja and eventually somehow obtained his permission. Also, that it was only after meting out the permission to other devotees that Baba sent word to Dada Kelkar. The expressions, "we were permitted", "Baba allowed…. quietly" in the above letter clearly indicate Baba's attitude toward the puja, suggesting that he would have almost unwillingly nodded his assent to it at the outset. [Ref: Arati Sai Baba, by Sri Babuji]
“One Gurupurnima day, numerous devotees came to Sai Baba, and as usual, placed a book before him so that he might return it with his asirvada or blessing for them to study it with profit and benefit. Sai Baba, however, took up a book brought by one man and gave it to another, as he often does.” [From Devotees’ Experiences of Shirdi Sai Baba, by B.V.Narasimhaswami]
"I give my devotees what they ask for until they ask for what I want to give." - Shirdi Sai Baba