Jaiminīya (Talavakāra) Brāhmaṇa 2-8.302

2-8.302

They became sharers of the days.
Then, day by day, one after another of them prospered: one on one day, one on one day, one on one day.
That, truly, is the best of prosperities, that is the holiest of worlds, which the Seven Ṛṣis prospered by this.
[Thinking,] 'May I prosper in that which is the best of prosperities, that which is the holiest of worlds, which the Seven Ṛṣis prospered by this,'
he therefore sacrifices with a seven-day rite.
These become seven days, seven stomas.
A stoma carries each day, a stoma each day, a stoma each day.
That, indeed, is the perfection of the days, that different stomas carry them.
That which is the perfection of the days is thus brought about.
And all seven of them, being combined, correspond to the twenty-one-fold [stoma]: the trivṛt and the trayastriṃśa make two twenty-one-folds; the pañcadaśa and the triṇava make two twenty-one-folds; the saptadaśa and the pañcaviṃśa make two twenty-one-folds; the twenty-one-fold is the twenty-one-fold itself.
Twelve months, five seasons, these three worlds: that sun is the twenty-first.
To companionship in the world of this [sun] go those who sacrifice with the seven-day rite.
Complying with this, their world prospers.
He goes to this, he attains companionship in the world of this, who knows thus.