Traditionally, Hindus celebrate Guru Purnima to mark the birth of Rishi Veda Vyasa on the full moon of Āṣāḍha month. Recently, there has been such digestion of Guru Purnima that it is slowly getting divorced from its Hindu roots.
Most of India's powerful leaders referred to it as the festival of Buddha. Veda-vyāsa was not even mentioned by many of them. In fact, India's President and PM called it “Dharmachakra day”.
As per this claim, it was on Guru Purnima (full moon of Āṣāḍha) that Buddha taught his first sermon at Sarnath in Varanasi. Through his first sermon, he is allegorically said to have “turned the chakra(wheel) of dharma”. The first sermon is called “Dhammacakkappavattana Sutta”.
Was Guru Purnima originally celebrated for Vyasa? Or Buddha? The Pali canon (Tripitaka) does not mention the DAY on which Buddha delivered his first sermon and “turned Dharma Chakra”. It is not mentioned at all in these early texts!
The day on which Buddha delivered his first sermon and “turned the wheel of dharma” is also not mentioned in Aśvaghoṣa's Buddhacarita (early 2nd century CE). When was it first mentioned? And what was the day? The earliest text that mentions the day on which Buddha taught his first sermon is a Buddhist text named Mahavastu. This text reached its final form between the 3rd-4th centuries CE. According to this text, the day was not Āṣāḍha Purnima/Guru Purnima. According to Mahavastu, the “Dharma Chakra day” was on the “twelfth day of the second fortnight of Āṣāḍha”. Other Buddhist texts also give the same date. In fact, Sikkim, which has a considerable number of Buddhists, officially celebrates this day on fourth of Āṣāḍha. Its name is their language is Drukpa Tsheshi. Drukpa means Āṣāḍha and Tsheshi means fourth.
Thus, it is clear that:
At a later date, Buddha's “Dharmachakra day” was (purposefully) juxtaposed with Guru Purnima/Vyāsa Purnima/Āṣāḍha Purnima. The appropriation has continued to such an extent that Guru Purnima is now being addressed as “Dharmachakra day” even in India. This appropriation must have been made many centuries ago. When “Dharmachakra day” was juxtaposed with Guru Purnima, this tradition was also taken to South East Asia. As a result, South East Asian countries also celebrate their “dharma day” on the full moon of Ashadha.