Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.3.1

2.5.3.1

varuṇapraghāsairvai prajāpatiḥ | prajā varuṇapāśātprāmuñcattā syānamīvā akilviṣāḥ prajāḥ prājāyantāthaitaiḥ sākamedhairetairvai devā vṛtramaghnannetairveva vyajayanta yeyameṣāṃ vijitistāṃ tatho evaiṣa etaiḥ pāpmānaṃ dviṣantam bhrātṛvyaṃ hanti tatho eva vijayate tasmādvā eṣa etaiścaturthe māsi yajate sa vai dvyahamanūcīnāhaṃ yajate

Verily, by means of the Varunapraghâsâh Pragâpati delivered the creatures from Varuna's noose; and those creatures of his were born without disease and blemish. Now with these Sâkamedha offerings,--therewith indeed the gods slew Vritra, therewith they gained that supreme authority which they now wield; and so does he now therewith slay his wicked, spiteful enemy and gain the victory: this is why he performs these offerings in the fourth month (after the Varunapraghâsâh). He performs them on two successive days.