BG 8.23-25
That time wherein going forth Yogîs return not, and also that wherein going forth they return, that time shall I declare to thee, O prince of the Bhâratas. Fire, light, day-time, the bright fortnight, the six months of the northern path—then, going forth, the men who know the Eternal go to the Eternal. Smoke, night-time, the dark fortnight also, the six months of the southern path—then the Yogî, obtaining the moonlight, returneth.