Well Said Sangom Ji,
Our Ancestors Strictly avoided Ashtami and Navami , Though we are Celebrating Gokulashtami, ( Lord Krishna's Birth ) and Sri Rama Navami (Lord Shri Ram's Birth),
This is because our Earth will be exactly at The Center and it will be Between Sun and Moon.
On those days The Sun will be pulling the Earth towards its Orbit and at The same Time Moon also will be pulling The Earth towards it.
As a result of This The Earth will Observe a Strong Vibration.
This will have impact on all the Living Things on Earth.
Because of this we will be in a Wavering Mentality at that time and Wont take a right decision.
So our ancestors avoided Ashtami and Navami.