Bheeshma breathed his last on Ashtami day which is commemorated as Bheeshma Ashtami, “Ashtami” meaning the 8th phase of the moon.
In Bheeshma Parva, in verses 6.114.86-100, after being mortally wounded, we find Bheeshma saying that he would wait until Uttarayana to die.
In Shanthi Parva, verse 47 -3, it is mentioned that Bheeshma breathed his last on the 8th phase in bright fortnight of Magha, i.e., on Magha Shukla Paksha Ashtami, now known as Bheeshma Ashtami.
We find that the winter solstice, Uttarayana, had occurred in lunar month of Magha, on Shukla Paksha Sapthami, 7th phase, brighter half.
Bheeshma breathed his last on the next day, Ashtami, 8th phase of the moon, 18th January, 3066 BCE.
The full details on the above can be read in our book, ‘Historical Krishna’.