Jaiminīya (Talavakāra) Brāhmaṇa 3.5

He who knows the self-rule of the meters goes to self-rule, he reaches the world of self-rule. The Bṛhatī is indeed the self-ruler of the meters. They perform it backward, forward, and across for food. Therefore they live upon cattle [moving] backward, forward, and across. Just so much indeed is the faculty and vital power in the year: namely the twelve-day rite, twelve full-moon days, twelve eighth days, twelve new-moon days. Truly, through this he secures that. He who knows the Virāj established in the seasons, and the seasons established in the Virāj, goes to a firm standing, he secures food. The Virāj has thirty syllables; the seasons are six. Thus indeed the Virāj stands firmly in the seasons, and the seasons in the Virāj. He goes to a firm standing, he secures food, who knows thus. In performing the twelve-day rite, it is truly this Anuṣṭubh that they perform. The Anuṣṭubh has thirty-two syllables. Cattle are four-footed. Speech is the Anuṣṭubh. Through speech he holds fast cattle. Therefore cattle, controlled by speech, summoned by speech, come.

The first three-day period has the Gāyatrī at its beginning.

Therefore this fire shines upward.

The second three-day period has the Gāyatrī in the middle.

Therefore this wind blows across.

The third three-day period has the Gāyatrī at the end.