Jaiminīya (Talavakāra) Brāhmaṇa 2-2.8

Then the ten twenty-five-fold chants are the Mahāvrata;
by them the Mahāvrata is undertaken by him on the Viṣuvant day.
Now the two stotriya verses: that is Āditya established on this day.
Now formerly they used to undertake the Mahāvrata three times: on the introductory day, on the Viṣuvant, and on the Mahāvrata day.
Of that introductory day there are fourteen twenty-five-fold chants and ten stotriya verses.
Then the fourteen twenty-five-fold chants are the Mahāvrata;
by them the Mahāvrata is undertaken by him on the introductory day.
Now the ten stotriya verses: the Virāj has ten syllables;
the Virāj is food;
with provisions, indeed, they go until the Viṣuvant day.
Now the two stotriya verses that are left over from the Viṣuvant: that is food and drink.
Twofold indeed is the form of food: that which one eats and that which one drinks.
Just as one would go eating and drinking, even so they go by means of these two until the Mahāvrata day.
They come to this heavenly world,
namely, this Mahāvrata day.
The heavenly world is indeed the Mahāvrata day.
He who knows thus—the Mahāvrata has been undertaken three times for him; he goes to the heavenly world.

Regarding this they say: 'tam īṃ hinvanty agruvaḥ' ('Him the maidens urge on') should be made the opening verse of this day.

'tam arkebhis tam sāmabhiḥ' ('Him with songs, him with chants') [is said] on account of the head; 'tad id āsa bhuvaneṣu jyeṣṭham' ('That indeed was the eldest in the worlds') on account of the body.

They proceed with this day as if saying 'tat tat' ('father, father').

From that [day] arose Prajāpati.

From that [day] arose this [father] of all deities.

This day is the father; the other days are the offspring.