Historical account of Janmasthami published in Anand Bazar Patrika, 25 Aug: English version
Krishna is definitely a fascinating subject for numerous scholars, whether religious or atheist, but he troubles historians the most. Contrary to normal belief, Krishna is not mentioned in the Vedas, and with great difficulty, we find his first undisputed mention as a character only in the Chhandogya Upanishad of the seventh century BC. He is also cited in the later Taittirya Aranyaka but there is no refernce to his birth-legend. This makes entry more than a thousand years later, in the Vishnu Puran and the Hari- vamsa of the 3rd or 4th centuries of the Christian Era.
In between, we do get some stray references in a few sacred narratives, but Krishna was certainly not portrayed as the great God that he became in later tradition. Janmashtami is not mentioned as a popular celebration and Krishna himself is completely overshadowed for several centuries by Vasudeva and BalaramaSankarsana. They were more powerful deities and Krishna was only their junior coalition partner till roughly the 4th century AD, ie, the Gupta period. Once Mahabharata and Gita came out in their final shape, Krishna emerged more prominently as the supreme Vaishnava challenge to rival Shivaism, and he subsumed both Vasudeva and Balarama into his own legend. We have a lot of evidence in Gupta and post-Gupta sculpture portraying the miraculous deeds of the divine child. Basically, they valorise him as the mascot of the new settled pastoral civilisation that rose on the banks of the Yamuna. Motifs like Krishna holding Govardhana hill over his head to protect his people against torrential rain is taken as evidence by eminent scholars like DD Kosambi and Jan Gonda (Yan Khonda), to represent the victory of the god of darker people over the hitherto-omnipotent god of wandering animal-grazing 'Aryan' tribes...............
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Source Credit and full story at : : http://jawharsircar.com/assets/pdf/Janmashatami_krishnajanmashtami_krishna_krishna_Birthday_gokulashtami_jawhar_sircar.pdf