Why are there so many variations in determining Srijayanthi? Why do we consider the association of Saptami for Srijayanthi and the association of Trayodashi for Narasimha Jayanthi? Why is it not calculated based on the existence of Rohini nakshatram during nighttime or the existence of Ashtami tithi?

We do not have the authority to determine when Ashtami occurs. Elders have determined this. Sri Sannidhi tradition says there must not be an association with Saptami. Similarly, they have determined that for Narasimha Jayanthi, the association of Trayodashi must not be more than 6 nazhigai-s. All these have been determined by the elders. We practice following these determinations. We are not Rishis or elders to determine these.