Bhishma left his body on the Ashtami day of Magh Shukla Paksha of Hindu month. This was the day, which is called the day of the gods. Bhishma Pitamah left his body on the auspicious day of Uttarayan, i.e. the day Lord Surya started moving towards north after completing the six-month period of Dakshinayana. It is believed that if a person dies during Uttarayan, he goes to heaven.
The anniversary of the death of Bhishma Pitamah is celebrated during Magha Shukla Ashtami. Hence the day is called ‘Bhishma Ashtami’, and unquestionably as the death anniversary of Bhishma Pitamah. According to the legend associated with that day, Bhishma waited for 58 days before leaving his body.