Jaiminīya (Talavakāra) Brāhmaṇa 3.2

He brought it.
He desired: 'Let the consecrated sacrifice me as a consecrated one.'
He said to the months and to the seasons: 'Sacrifice for me!'
The months did not speak to him:
'You shall indeed enjoy her [nourishment] when she is present only in us.'
Therefore, all these full-moon days, aṣṭakā days, and new-moon days belong only to the months.
Then, once the soma was pressed, the seasons realized behind [the months]: 'He has indeed given to them!'
They said to him: 'Give the sacrificial fee to us as well!'
To them he gave food.
That is why food ripens according to the season.
Through the consecrations he obtained the full-moon days; through the upasads, the aṣṭakā days; through the pressing, the new-moon days.
Whoever, knowing thus, sacrifices with the twelve-day rite obtains this very sacrificial nourishment, invigorating draught, food, and imperishability.
The seasons accepted; the months did not desire acceptance [of the fee].
Therefore, the months are as if more celebrated.
For one who does not accept becomes more celebrated.
Therefore, the seasons are the elder.
For one who accepts becomes the elder.
These indeed are the three nights: full-moon days, aṣṭakā days, and new-moon days.

In the beginning, the gods did not submit to Indra for supremacy.

He ran up to Prajāpati: 'Grant these to me!'

To him he granted these two: the full-moon day and the new-moon day.

In himself he retained the aṣṭakā day.