Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.2.5

8.3.2.5

atha 'rtavyā upadadhāti | ṛtava ete yadṛtavyā ṛtūnevaitadupadadhāti nabhaśca nabhasyaśca vārṣikāvṛtū iti nāmanī enayorete nāmabhyāmevaine etadupadadhāti dve iṣṭake bhavato dvau hi māsāvṛtuḥ sakṛtsādayatyekaṃ tadṛtuṃ karotyavakāsūpadadhātyavakābhiḥ pracādayatyāpo vā avakā apastadetasminnṛtau dadhāti tasmādetasminnṛtau bhūyiṣṭhaṃ varṣati

He then lays down two Ritavyâ (seasonal bricks);--the two seasonal ones being the same as the seasons, it is the seasons he thus places therein.[Vâg. S. XIV, 15], 'Nabha and Nabhasya, the two rainy seasons,' these are the names of those two (bricks): it is by their names he thus lays them down. There are two (such) bricks, for a season consists of two months. He settles them once only: he thereby makes (the two months) one season. He places them on avakâ-plants and covers them with avakâ-plants; for avakâ-plants mean water he thus bestows water on that season, whence it rains most abundantly in that season.