This fast starts from Kartik Shukla Ekadashi and continues till full moon. Women and men who take bath in Kartik observe fast by staying fast. This day is named after Bhishma Pitamah. When Lord Krishna and the five Pandavas had gone to Bhishma to receive state-related teachings, Bhishma preached for five days and Lord Krishna was very pleased with the teachings of Pitamah Bhishma. Shri Krishna named these five days as Bhishma Panchak Vrat.
Lord Krishna is worshiped with the mantra 'Om Namo Bhagwate Vasudevaya'. Ghee lamp should be kept on continuously for five days. Havan should be performed by offering 108 offerings of ghee, sesame and barley with the mantra 'Om Vishnuv Namah Swaha'.
At the end of the war of Mahabharata, when Bhishma Pitamah was sleeping on the bed, waiting for the sun to be Uttarayan. Then Lord Krishna took the five Pandavas with him and went to them. Knowing the opportune moment, Yudhishthira requested Bhishma Pitamah to please give us instructions regarding the kingdom. Then Bhishma Pitamah preached for five days on Raj Dharma, Varnadharma, Mokshadharma, etc. Hearing his teachings, Shri Krishna was satisfied and said, 'Father! I am very happy with the righteous preaching you have given in the five days till the full moon in Kartik Shukla Ekadashi. In his memory, I establish Bhishma Panchak fast in your name.'
Those who do this, they will attain salvation in the end after enjoying various pleasures throughout their life.