Jaiminīya (Talavakāra) Brāhmaṇa 37.219

That is the nature of the Viśāla. This is a sāman that brings heaven. He attains the heavenly world who knows thus. Just as, indeed, there are the steps of the sun, so these are the steps of the Viśāla for the attainment of the heavenly world. He reaches the heavenly world who knows thus. Now the Rathantara. The Rathantara is the brahman. They go establishing themselves in the brahman. It is on the Traiṣṭubha day. What is the Triṣṭubh is the form of the male; the Rathantara is that of the female. Since the Rathantara is the pṛṣṭha here, thereby this day is made a mating pair. Now the Vāmadevya. Cattle indeed are the Vāmadevya. Rich in cattle they become who praise with this. 'Whether, O Indra, from the east, from the west, from the north—' thus they call Indra from all directions, when the days have been completed: 'Whether in this direction you are, whether in this, whether in this.'

Upon them [is sung] the Vāśa having the word-finale, [which belongs to] the Rathantara.

Therefore it is performed on the Rathantara-day.

Those gods desired: 'May we appropriate the will of the Asuras!'

They saw this sāman.