Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.2.6

8.3.2.6

athottare | iṣaścorjaśca śāradāvṛtū iti nāmanī enayorete nāmabhyāmevaine etadupadadhāti dve iṣṭake bhavato dvau hi māsāvṛtuḥ sakṛtsādayatyekaṃ tadṛtuṃ karotyavakāsūpadadhātyāpo vā avakā apastadetasya 'rtoḥ purastāddadhāti tasmādetasya 'rtoḥ purastādvarṣati nopariṣṭātpracādayati tasmānna tathevopariṣṭādvarṣati

Then the two upper ones, with (Vâg. S. XIV, I6), 'Isha and Ûrga, the two autumnal 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 only once: he thereby makes (the two months) one season. He places them on avakâ-plants, for the avakâ-plants mean water: he thus bestows water before that season, whence it rains before that season. He does not cover them afterwards, whence it does not likewise rain after (that season).