Shatapatha Brahmana 8.2.1.16

8.2.1.16

atha 'rtavye upadadhāti | ṛtava ete yadṛtavye ṛtūnevaitadupadadhāti śukraśca śuciśca graiṣmāvṛtū iti nāmanī enayorete nāmabhyāmevaine etadupadadhāti dve iṣṭake bhavato dvau hi māsāvṛtuḥ sakṛtsādayatyekaṃ tadṛtuṃ karoti

He then lays down two Ritavyâ (seasonal bricks);--these two, the Ritavyâ, are the seasons (ritu): it is the seasons he thus bestows thereon. [He lays them down, with Vâg. S. XIV, 6], 'Sukra and Suki, the two summer-seasons;'--these are the names of these two: it is with their names that he thus lays them down. There are two bricks, for a season consists of two months. He settles them once only: he thereby makes (the two months) one season.