Celebrated in Magh Shuki Ashtami, is in reverence to Bhisham. In addition to Bhisham Pitamah of Mahabharata, fast as Bhisham Panchak (Kartik Shuki Ekadashi), Bhisham Ashtmi is observed in reverence for the occasion when Bhisham finally died. In respect to Bhisham, Krishna had to break his promise.
Mahabharata describes that Bhisham was the son of King Santanu by his wife Ganga. In order to please his father, Bhisham took an oath not to marry and that he would ever remain faithful to the throne of Hastinapur (Kaurav dynasty to Dhritrashtra, Duryodhan etc.). During the battle of Kurukshetra, Bhisham was obliged to enter the war.
As there were defeat after defeat, Duryodhan suspected the partial behaviour of Bhisham. Bhisham, to save his self respect, in anguish, took up such an arm, which could only be opposed by Krishna, but Krishna at the beginning of the war, had declared that he would not lift a weapon during the war. But since Bhisham's attack could not be counteracted except by Krishhna, the latter had to break his promise and was compelled to lift the Sudarshan Chakra. Seeing that, Bhisham stopped fighting and laid himself to death on the bed.
This day is observed as a festival in reverence to Bhisham.