Magha Suddha Astami aka Bhishmastami is the day when Bhishma attained nirvana. During the 18-day battle of Mahabharatha, when Bhishma, the fierce warrior, blinded by duty, neglecting his virtue, kept on thundering on the Pandava army, on the 11th day of battle, Arjuna defied him with his skill, motivated by Krishna.
As Bhishma realised his mistake of taking the unrighteous side in the battle, he stayed paralysed on the arrow bed until everything is in Pandava control. Bhishma has the ability to leave his body at will, but he couldn’t do it as he got so heavy in heart and had to wait for longer until Uttharayana. As Bhishma attained God-realisation, he uttered Vishnu Sahasranama! His mother, Ganga, sent for him a heavenly abode: a sky chariot with the likeness of swans, where he returned to his source on this day.
It is a not just a mere coincidence where the festival Rathasaptami is also marked on this very same day. People not only contemplate the immortality of their being like the sun, and they also think of Sri Vishnu Sahasranamas and of Bhishma’s realisation, on this day!
– Venkat Dasari