We have written about two Janmashtami dates even within the same city Mathura, the birthplace of Lord Krishna. We have highlighted the influence of ISKCON and Vaishnava Sampradaya due to which most people celebrate Janmashtami according to Vaishnavite rules.
If one is not the follower of Vaishnavism then one should not follow Vaishnava date of Janmashtami and should observe Janmashtami as per Smarta rules. But unfortunately many people don’t understand this difference.
In Mathura Prachin Keshava Dev Mandir (which should not be confused with newly constructed temple with similar name at Janmabhumi), which claims to be the most authentic place of Lord Krishna’s birth, celebrates Janmashtami according to Smarta rules.
Shri Krishna Chandra Goswami from the old Keshava Dev Mandir says "We don’t care when other temples are celebrating Janmashtami and we always follow it on Saptami Tithi (usually Ashtami prevails during midnight of Saptami Tithi) while Ashtami prevails during midnight". In other words old Keshava Dev Mandir celebrates Janmashtami according to Smarta Sampradaya and is not influenced by ISKCON and other prestigious temples in Mathura and Vrindavan founded by Vaishnavite.
It is important to understand the difference between Smarta and Vaishnava to choose right Janmashtami date. In Hinduism there are three major sects namely Vaishnavism, Shaivism and Shaktism which revere only Vishnu, only Shiva and only Shakti respectively. These are non-liberal sects and believe in the supremacy of single deity.
If you worship all Gods and believe that all deities are manifestation of one God then you belong to Smarta Sampradaya. Smarta Sampradaya is a liberal and nonsectarian group which accepts all major Hindu deities as form of one supreme God.
After reading above explanation it should be easy to choose the school one belongs to. If you belong to Smarta School then you should not follow ISKCON date of Janmashtami. We emphasize on Smarta date because majority of Hindus believe in multiple God. We also make a note on Vaishnavite date of Janmashtami if it falls on the different date.
In 2012 most temples in Mathura and Vrindavan celebrated Janmashtami on 10th August except the old Keshava Dev Mandir which stuck to its old tradition and celebrated Janmashtami on 9th of August. In 2013 there would be single date of Janmashtami for both Sampradaya. But in 2014 again there would be two dates of Janmashtami.