Jaiminīya (Talavakāra) Brāhmaṇa 41.232

He saw this sāman. With it he propitiated him: "I praise your dearest guest, beloved like a friend; like a chariot, Agni is to be known." Thus indeed he made him the dearest guest. He is, as it were, unpropitiated at this point, having borne the sacrifice. They propitiate him here at this moment. He, propitiated, carries whatever remains of the sacrifice here. Because Uśanas Kāvya saw it, therefore it is called the Auśana. "O Indra, come to us, dear one, winning everything, concealed from none, broad on all sides like a mountain, lord of heaven! For [you are] truthful, soma-drinker!" In that there is "abhi" (unto), that is the form of the Rathantara. This day belongs to the Rathantara. "You became both rodasi (heaven and earth)" — these two indeed are heaven and earth; it is these two that it addresses. "Indra, you are the increaser of him who presses [soma], lord of heaven" — for this forty-fourth day is increased.

On these [verses], the Sāṃvarta [sāman] with word-final finale (padanidhana) belongs to the Rathantara.

Therefore it is performed on the Rathantara day.