A great warrior, a man of his words and an inspiring leader, Bhishma Pitamah was not just a great character in the Mahabharata, but also became a historical figure who will continue to be remembered for ages and forever. The uncle of the Pandava and Kaurava brothers, Bhishma Pitamah was the son of king Shantanu and Ganga. He is mainly remembered for the great role he played in the Mahabharata.
Bhishma Ashtami is the day when he departed from his body.
It is said that his father Shantanu had actually given him the boon of Iccha Mrityu. It translates to 'death as per will'. According to it, he had the power to leave his body as per his will. Nothing and nobody could actually kill him against his wish.
However, despite being one of the most responsible, experienced and learned members in the entire clan, Bhishma decided to participate in the war from the Kauravas' side. The decision was justified by him later on the death bed by saying that since he had been staying with the Kauravas and eaten their salt, he was in debt to pay them back in some way. Hence, he fought as a Kaurava warrior.
When Arjuna attacked him in the Mahabharata, while Bhishma's attention was caught by the reincarnated Shikhandi, the attack made injured him badly. He lay blood soaked on a bed of arrows. However, he had the boon not to die against his wish. So despite being badly injured, he decided to die only when the Sun was Uttarayan, that is moving northwards. It is said that who dies during the Uttarayan period goes to heaven after death. Hence, he waited for 58 days for leaving his body.
In the honour of Bhishma Pitamah, people celebrate Ekodishta Shraddha, a ritual done for a person sent to heaven after death.
People perform the ritual of Tarpan, in order to offer prayers for the peace of Bhishma Pitamah's soul. They also perform this ritual for their own departed forefathers.
People take bath in the river Ganga and make an offering of boiled rice and sesame seeds. It is said that doing so liberates them from the cycle of birth and death.
Some people even observe a fast on this day, perform Arghyam and chant Bhishma Ashtami Mantra.