Why are some occasions reckoned using nakshatras while others using tithi? Like one's birth is usually marked using nakshatra, but devasam goes by tithi.
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.
vikrama, SVs use both tithi and nakshatram for determining most festivals (Deepavali is a notable exception, it is strictly by tithi). The most complicated formula is Sri Jayanthi, the rule to determine Sri Jayanthi will make even Aryabhata's head swim.....Regarding Krishna Jayanti, Vadamas celebrate it on Astami and Vaishnavas on Rohini. This may be due to the vaishavas being the original inhabitants of Tamilnadu and Vadamas being the immigrants from north as their name suggests.