Bhishma Dwadashi Fast Story

During the Mahabharata war, Bhishma Pitamah fought on the side of the Kauravas. The Pandavas realized they could not defeat the Kauravas as long as Bhishma was fighting for them. They also learned that Bhishma had taken a vow never to raise weapons against a woman. Using this to their advantage, the Pandavas sent Shikhandi, who was born as a woman but later became a man, to face Bhishma in battle. Seeing Shikhandi, Bhishma, due to his vow, put down his weapons. Arjuna seized the opportunity and struck Bhishma with countless arrows, wounding him gravely. Bhishma fell onto a bed of arrows but did not give up his life because he had the boon of "Ichha Mrityu" (the ability to choose the time of his death) given by his father, King Shantanu. Bhishma waited until the Sun shifted from the Dakshinayan (southern course) to the Uttarayan (northern course). On the Ashtami Tithi (eighth lunar day) of Uttarayan, Bhishma gave up his life. However, his final rites were performed on the Dwadashi Tithi (twelfth lunar day) of the Shukla Paksha (waxing moon phase) in the month of Magh. Since then, this day has been observed as Bhishma Dwadashi.