The story
Bhīṣma lay on a bed of arrows at Kurukṣetra, holding off his own death because he could choose the hour of it. The war was over and Yudhiṣṭhira came to him with questions about duty, and the old man answered for days.
Asked what was the surest and shortest path, he gave the thousand names of Viṣṇu — the Viṣṇu Sahasranāma. It is the single best-known Sanskrit recitation in Vaiṣṇava practice, and it is read end to end today.
He waited for the northward turn of the Sun before letting go. The eighth day of the same bright half, Bhīṣma Aṣṭamī, is kept as the day he died, and water is offered for him by name even by those who are not his line.