Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.29

6.2.2.29

tadāhuḥ | yadyāvatya etasyāgneriṣṭakāstāvanti kraye 'horātrāṇi sampadyante 'tha yānyūrdhvāni krayādahāni kathamasya te lokā anūpahitā bhavantīti yadvā amāvāsyāyāṃ dīkṣitvāmāvāsyāyāṃ krīṇāti tadyāvantameva lokaṃ karoti tāvatīriṣṭakā upadadhātyatha yānyūrdhvāni krayādahāni tasminnavakāśe 'dhvaryuragniṃ cinoti kvo hi cinuyānna ca so 'vakāśaḥ syādyāvanti vai saṃvatsarasyāhorātrāṇi tāvatya etasyāgneriṣṭakā upa ca trayodaśo māsastrayodaśo vā eṣa māso yānyūrdhvāni krayādahāni tadyā amūstrayodaśasya māsa iṣṭakāstābhirasya te lokā anūpahitā bhavanti tatsamā lokāśceṣṭakāśca bhavanti

As to this they say, 'If at the time of the buying (of Soma) the days and nights (of the initiation-period) amount to just as many as there are bricks of that fire-altar, why then are not those spaces of his filled up (which are prepared) during the days there are after the buying (of Soma)? Well, when he buys (Soma) at new moon, after becoming initiated at new moon (a year previously), then he piles up just as many bricks as (during that interval) he makes space for; and what days there then are after the buying (of Soma), during that interval the Adhvaryu builds up the fire-altar. But when should he build up, if there were not that interval? As many as there are days and nights in the year, so many are the bricks of that fire-altar. Thereto (comes) a thirteenth month, for there is that thirteenth month;--thus during the days there are after the buying (of Soma), those spaces of it (the altar) are filled up afterwards with those bricks of the thirteenth month: thus the spaces and the bricks become equal.