Uttarayan marks the beginning of longer days on account of the sun's northward journey & it is believed that someone who dies on this day, is granted 'moksha'.
Let us know a little more about the death of the grandsire of the Kuru princes, Bhishma Pitamah and the story of his death, which happens on Uttarayan.
Vichitravirya, the prince of Hastinapur and the step-brother of Bhishma had become of marriageable age. Bhishma, as the protector of the realm, took it upon himself to find a suitable wife for him.
There was a wavayamvara ongoing by the king of Kashi for his daughters Amba, Ambika and Ambalika. Bhishma rode to the palace, announced his intention of abducting the three princesses back to Hastinapur.
The princes present there fought him but Bhishma defeated them all. He even fought a duel with the king of Shalwa, whom he defeated very easily as well.
When the group finally reached Hastinapur and the preparations for the wedding began, Amba shored up her courage and mentioned her love for the king of Shalwa, whom she intended to marry in the swayamvara.
Bhishma found this a valid reason and let her go. He even arranged for her to reach the king.
However the Shalwa king refused to take her back, insisting that Bhishma had won her fair and square. She pleaded with him, but he did not budge. Sad, she went back to Bhishma and told him what happened. Bhishma asked Vichitravirya to marry her, but he refused saying she loved another man. Amba then pleaded with Bhishma to marry her but he too refused owing to his vow of celibacy.
Enraged at the three consecutive refusals, she appealed to various kings to fight Bhishma for her, whom she considered the main culprit for her plight, but no one accepted for fear of Bhishma's power.
She finally went to the forest to please the gods for help. There she came across Parashurama, who agreed to fight his disciple Bhishma.
But the fight was a stalemate, and so Amba was yet again left unavenged. Disappointed yet again, she now began strict penance and managed to please Shiva for a boon, to be born as a man and kill Bhishma. After getting the boon, she created a pyre and jumped in to it, to end this life and take birth again.
Years later, during the Kurukshetra war, Amba faced Bhishma as Shikhandi, whom Bhishma refuses to fight, for he knew her to be a woman, and lowered his weapons. Arjuna, standing behind Shikhandi, greeted Bhishma with a volley of arrows and that is how the great Bhishma ended up on a bed of arrows in the war.
Bhishma had been blessed with the ability of 'iccha-mrutyu', i.e. he could choose the time of his death.
After lying on the bed of arrows for a couple of days, he eventually gave up his life on the day of Uttarayan.