If Ashtami and Navami are considered as bad, then why Lord Krishna and Lord Rama born on these days?
Ashtami and Navami are not bad days. During the days when there was no clock and calendar, the schools (gurukul) had a way to count the days for conducting classes and have weekends too, in between. Their logic of calendar is to count the days starting from either full moon day or new moon day. New moon and full moon both have 15 days in between. So, they start counting right after full moon or new moon.
You know a truth. Rama, Krishna, and Jesus they were not born through YONI. It is not prasava, but pravesa means entering. That is why we say Virgin Mary, and Ram, Lashman, Bharath and Shatrugan were born through the milk that was provided by the Agni deva [Fire god], Krishna born with Maya and you know the stories.
In some parts of North India merchants select this days for new account opening or new business starting, with the hope as it will prolong on. Their birth horoscope is the best one so these days are side lined, just like a good lucky person living in a wrong vasthu house.