The Rudra rule this day, which is good for taking up arms, building of one's defenses, and fortification.
In olden days the astami and navami are the days which comes in-between if you see any work its auspicious to start in amavasa or the next day and the work can be done for next 7 days where the moon will not be so bright. Hence they can not go to the temple and offer prayer etc., Hence the work for first 7 days after amavasa after that the moon will be brighter so they can do some work in night. Hence they're not prefer astami and navami for starting any job because they can complete major work if the start after amavasa . This is the same reason why they don't start any work from pournami also. But slowly those 2 days are treated as unauspicious days. Because no current is there during those days and all live with nature unlike current days
Krishna Janmashtami is observed on the Ashtami tithi, the eighth day of the dark half or Krishna Paksha of the month of Bhaadra in the Hindu calendar, when the Rohini Nakshatra is ascendant. Rasa lila or dramatic enactments of the life of Krishna are a special feature in regions of Mathura, and Vrindavan, Nalbari and regions following Vaishnavism in Manipur.