About Two Krishna Janmashtami Dates Most of the time :

Rules for Krishna Ashtami nirnaya are quite complex which are explained in Dharma Bindu: page 8-9 , DHARMA SINDHU: Prathama Parichheda and Nirnaya Sindhu: Dwitya Parichheda etc in detail. (links in last tweet)

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The basic summary of the rules is,

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).

If this auspicious combination happens then, all celebrate the Krishna Janmashtami on the same day.
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.

A single date for Janmashtami means that both Sampradaya would observe Janmashtami on the same date.

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.

The International Society for Krishna Consciousness, commonly known as ISKCON is founded on the principles of Vaishnava traditions and most followers of the ISKCON are the followers of Vaishnavism.

With all due respect, ISKCON is one of the most commercialized and global religious institutions which spend money and resources to promote ISKCON brand and ISKCON culture.

In North India, most people observe Janmashtami on the day chosen by ISKCON.

Many people who are not the followers of Vaishnavism do not even understand that ISKCON traditions are different and the most appropriate day to observe Janmashtami fasting might not be the same as that of ISKCON.

Smarta followers who understand the difference between Smarta and Vaishnava sectarian do not follow ISKCON date to observe Janmashtami fasting. Unfortunately, ISKCON date to observe Janmashtami is unanimously followed in Braj region

.. and most common people who just follow the buzz observe it on the date followed by the 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

and those rules should be followed to decide Janmashtami day if one is not the follower of Vaishnava Sampradaya. Ekadashi fasting is one of the good examples to understand this difference.

Rules to observe Ekadashis' fasting are also different for Smarta and Vaishnava communities. However, there is more awareness about different Ekadashi rules followed by Vaishnava sectarian.

Not only Ekadashis, Vaishnava fasting day for Janmashtami and Rama Navami might be one day later than Smarta fasting day. The followers of Vaishnavism give preference to Ashtami Tithi and Rohini Nakshatra. The followers of Vaishnavism never observe Janmashtami on Saptami Tithi.

Janmashtami day according to Vaishnava rules always fall on Ashtami or Navami Tithi on Hindu calendar. However, rules followed by Smartism to decide Janmashtami day are more complex. The preference is given to Nishita Kaal or Hindu midnight.

The preference is given to the day, either Saptami Tithi or Ashtami Tithi, when Ashtami Tithi prevails during Nishita and further rules are added to include Rohini Nakshatra.

The final consideration is given to the day which has the most auspicious combination of Ashtami Tithi and Rohini Nakshatra during Nishita time.

Wish you A happy Jansmashtami To All :) https://www.drikpanchang.com/dashavatara/lord-krishna/krishna-janmashtami-date-time.html?year=2020

Post by Jin Thakur at Hinduism SE