The most auspicious day to celebrate Krishna Janmashtami is when Bhadrapada Krishna Paksha (in Purnimanta calendar) Ashtami Tithi and Rohini Nakshatra occurs during Nishita Kaala (Hindu midnight).
Krishna Janmashtami is listed on 2 consecutive days. The 1st is for Smarta Sampradaya & the other one is for Vaishanava Sampradaya. Vaishanava Sampradaya date is the latter one.
However many people will notice unanimity in North India on choosing the day to celebrate Krishna Janmashtami. The reason behind this unanimity is the institution of ISKCON.
In North India, most people observe Janmashtami on the day chosen by ISKCON.
People who are not the followers of Vaishnavism are followers of Smartism. Hindu religious texts like Dharmasindhu and Nirnaysindhu have well-defined rules to decide Janmashtami day
Wish you A happy Jansmashtami To All :)
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