Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.30

6.2.2.30

etadvai yaiva prathamā paurṇamāsī | tasyām paśumālabhate yā prathamāṣṭakā tasyāmukhāṃ sambharati yā prathamāmāvāsyā tasyāṃ dīkṣata etadvai yānyeva saṃvatsarasya prathamānyahāni tānyasya tadārabhate tāni ca tadāpnotyathātaḥ sampadeva

Thus, then, what first full moon there is (in the year) on that he slaughters the victim; and what first eighth-day there is, on that he prepares the fire-pan; and what first new moon there is, on that he becomes initiated: thus whatever first days there are in the year, of those he thereby takes possession for him (Agni, the altar), those he thereby gains. Now then as to the total amount (of the fire-altar).