When the final attempts to make peace between the Kauravas and Pandavas fail, it leads to the Kurukshetra war, which was fought for a long time. Even though Pandavas were on the side of Dharma, they were unable to win because of Bhishma.
Dhramaraja approaches Bhishma and seeks his advice to win the war as the war being fought by them is on the righteous side . Bhishma tells Dharmaraja to keep Shikandi in front of Arjuna and fight. He tells him he will not fight against Shikandi.
Arjuna fights with Bhishma with Sikandi in front. Bhisma gives up the fight and he is pierced by Arjunas arrows all over the body and Bhisma falls down the ground and lies on the arrows as his bed . He was able too do so because of his strength .when his head was hanging Arjuna shoots some more arrows and arranges a pillow for his head.
When Bhishma asks for water Arjuna hits the ground with his arrow and gets the Ganga water to quench Bhismas thirst and Ganga being his mother takes care of him.
When Bhisma was in this condition Lord Krishna brings the Pandavas to him and requests Bhishma to teach the righteous things in life, like how the King should rule, how the ministers should be etc.
Bhishma before his death reveals to the Pandavas that Krishna is none other than Lord Vishnu himself and recites 1000 Verses in the prise of Lord Vishnu which we all now know as Vishnu Sahasranaamam. It is a very sacred and many people recite it everyday.
As Bishma has the boon to choose his day of death he decides to leave this world and join the other 7 VASUS who returned to the Celestial world immediately after birth. Bishma died on Ekadasi which the 11th day after new moon. This day is known as Bhisma Ekadasi. This day is considered as a sacred day by all Hindus.