Bhishma Dwadasi, or Bhishma Dvadasi, is observed on the 12th day in the month of Magha (January – February) during the Shukla Paksha (waxing phase of moon). Bhisma Dwadasi is also known as Magh Shukla Dwadasi. It is widely believed that Pandavas performed the last rites of Bhishma, the son of King Shantanu and Ganga in Mahabharata, on this day.
It is popularly believed that Bhisma who was mortally wounded and was lying on a bed of arrows chose to leave the earth 58 day after the beginning of the Great War in Mahabharata. Bhishma was blessed with a boon that he can choose the day of his death. It is believed that he chose Magh Shukla Ashtami day to leave the earth and merge with Brahman.
On the Bhishma Dwadasi day, Pandavas performed the last rites of Bhishma Pitamah on the banks of Ganga.There is a popular belief that offering Tarpan and Shradh on the day to ancestors is highly beneficial. Some people also perform Tarpan in the name of Bhishma as he had no children.