Jaiminīya (Talavakāra) Brāhmaṇa 37.217

Now the Hārāyaṇa. The gods and the Asuras were contending. Those gods desired: 'May we take away the fury of the Asuras!' They saw this sāman. They praised with it. By it, they took away the fury of the Asuras. In that they took away the fury, that is the nature of the Hārāyaṇa. He takes away the fury of his hateful rival who knows thus. Hārāyaṇa Āṅgirasa, desiring cattle, performed asceticism. He saw this sāman. He praised with it: 'Be powerful, O lord of power, Indra, with all [your] aids! / For we pursue you, the glorious finder of wealth, like Bhaga, O hero! / Abounding in horses, a maker of much cattle you are, a golden spring, O god.' Just so he obtained cattle consisting of cows and horses. Having sacrificed with those cattle, he went to the heavenly world. This is a sāman that brings cattle and brings heaven. He obtains cattle, he goes to the heavenly world, who knows thus. And because Hārāyaṇa Āṅgirasa saw it, it is called Hārāyaṇa.

Now the Achidra having the word-finale, [which belongs to] the Rathantara.

Therefore it is performed on the Rathantara-day.