Rules for Krishna Ashtami nirnaya are quite complex which are explained in Dharma Bindu: page 8-9 (pdf might get downloaded by clicking on this link), DHARMA SINDHU: Prathama Parichheda and Nirnaya Sindhu: Dwitya Parichheda etc in detail.
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.
If this combination doesn't happen like this year:
Ashtami Tithi Begins - 08:09 AM on Aug 23, 2019
Ashtami Tithi Ends - 08:32 AM on Aug 24, 2019
Rohini Nakshatra Begins - 03:48 AM on Aug 24, 2019
Rohini Nakshatra Ends - 04:17 AM on Aug 25, 2019
Then, few would celebrate by giving more preference to Ashtami Tithi and few would celebrate by giving more preference to Rohini Nakshatra.
As mentioned in this article:
तिथि के हिसाब से जन्माष्टमी 23 अगस्त को मनाई जाएगी. वहीं, रोहिणी नक्षत्र को प्रधानता देने वाले लोग 24 अगस्त को जन्माष्टमी मना सकते हैं
Who give preference to Ashtami Tithi would celebrate on 23rd Aug while who give preference to Rohini Nakshatra would celebrate on 24th Aug.
Why have we celebrated Dussehra today instead of tomorrow?