Today is 14 January 2021, the day of Makar Sankranti.
Today is also being celebrated as Uttarayan (movement towards Uttar), meaning that the Sun has reached its southward point and its Northward movement has begun.
Sankranti and Uttarayan are two distinctly phenomenon and should not be mixed. Makar Sankranti is Transit of Sun from Dhanu to Makar Rashi, and Uttarayan is Winter Solstice or its northward movement. Therefore, the day of Makar Sankranti should not be confused with Winter Solstice.
It is recorded in Mahabharat that the epic war happened in 3137 BC. After being hit Bhishma Pitamah he waited for Uttarayan to leave his mortal body, and in his memory, we still observe Bhishma Ashtami on Magh Shukla Ashtami and do tarpan for him.
At the time of Mahabharat war Bhishma was 125 years old and Krishna was 90 years old. At that ripe age, Bhishma was infallible in the war, and so Krishna told the the Pandavas to go to Bhishma and ask him a way to defeat him. Such was the magnanimity of the great warrior that he told it to them, and it was upon his advise the Pandavas brought in Shikhandi and Arjun started shooting his arrows while standing behind Shikhandi, and Bhishma not once did he look towards Shikhandi. When Bhishma fell down the war stopped for the day. His body was pierced by the arrows of Arjyun, but he did not die because he had the boon of choosing the time of his death. Bhishma waited for 58 days for the auspicious time to Uttarayan to start to leave his body.
The day Bhishma left his mortal body it was Magh Shukla Ashtami.
Magh Shukla Ashtami never coincides with Makar Sankranti. In the year of Mahabharat Makar Sankranti was on 2 November and Bhishma Ashtami was on 28 November, which shows Bhishma was waiting for Uttarayan and not for Makar Sankranti.
In 2021 Makar Sankranti is on 14 Janary and Bhishma Ashtami is on 20 February. These dates have shifted from November to February due to Earth's precession.