Actually Bhishma's time of death is not considered auspicious, it is just that on an auspicious time Bhishma left his body as he had the boon of ichha mrityu (death at will). And in remembrance of that this specific day is celebrated.
There are these two paths known as *pitru yāna * and deva yāna through which an eligible jivatma goes to the upper worlds after leaving the body. Of these two deva yāna leads to the world of Brahman reaching where one doesn't return and pitru yāna leads to the world of manes or chandra loka from where one has to return after the end of the merits.
When the sun is ascending the northern course in the sky it is called as Uttarayana which opens the path of deva yāna and when the sun is moving southern it is called as Dakhinayana opening the path of pitru yāna. Shri Krishna describes these two paths as below in the Gita:
agnir jyotir ahaḥ śuklaḥ ṣaṇ-māsā uttarāyaṇam
tatra prayātā gacchanti brahma brahma-vido janāḥ [BG - 8.24]
Those who know the Supreme Brahman attain that Supreme by passing away from the world during the inﬂuence of the ﬁery god, in the light, at an auspicious moment of the day, during the fortnight of the waxing moon, or during the six months when the sun travels in the north.
dhūmo rātris tathā kṛṣṇaḥ ṣaṇ-māsā dakṣiṇāyanam
tatra cāndramasaṁ jyotir yogī prāpya nivartate [BG - 8.25]
The mystic who passes away from this world during the smoke, the night, the fortnight of the waning moon, or the six months when the sun passes to the south reaches the moon planet but again comes back.
The yogis take the first path and reaching the abode of Brahman they do not return here again. Bhisma was aware of this. Hence even though he fell down shot by arrows on Margasirah Krishna Saptami, he waited till Magha Sukla Ekadashi to leave his body. Because only then that the auspicious moment for leaving the body through deva yāna was going to open.
Regarding planetary positions I don't think these two paths are dependent on them because they cannot be same every year where as such movement of Sun happens every year. The paths have something to do with the day, month, year and sun as described in the Chandogya Upanishad:
Those who know this and those who with faith and austerity worship in the forest – they go to light; from light to the day, from the day to the bright fortnight (sukla paksha), from the bright fortnight to those six months during which the sun travels northward (uttarayana); from the months to the year, from the year into the sun, from the sun into the moon and from the moon into the lightning. There exists a non-human person, who leads them to attain Brahman. This is the path of the gods (deva yāna). [Chg. Up. - 5.10.1,2]