The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com

20. And for the same reason (the departed soul follows the rays) also during the southern progress of the sun.

For the same reason, viz. because waiting is impossible, and because the fruit of knowledge is not a merely eventual one, and because the time of death is not fixed, also that possessor of true knowledge who dies during the southern progress of the sun obtains the fruit of his knowledge. Because dying during the northern progress of the sun is more excellent, and because Bhîshma is known to have waited for that period, and because scripture says, 'From the light half of the month (they go) to the six months when the sun goes to the north,' it might be thought that the northern progress of the sun is needful for dying. This notion the Sûtra refutes. The greater excellence of the sun's northern progress applies to those only who do not possess the highest knowledge.--Bhîshma's waiting for the sun's northern progress was due to his wish of upholding good customs and of showing that by the favour of his father he could choose the time of his death.--And the sense of the scriptural passage quoted will be explained under IV, 3, 4.--But we have the following Smriti-text, 'At what times the Yogins depart either not to return or to return, those times I will declare to thee' (Bha. Gîtâ VIII, 23), which determines specially that to die by day and so on causes the soul not to return. How then can he who dies by night or during the sun's southern progress depart not to return? Concerning this point the next Sûtra remarks: