Verse 1.12

मासो वै प्रजापतिस्तस्य कृष्णपक्श एव रयिः शुक्लः प्रणस्तस्मादेत ऋषयः शुक्ल इष्टं कुर्वन्तीतर इतरस्मिन् ॥ १२ ॥

māso vai prajāpatistasya kṛṣṇapakśa eva rayiḥ śuklaḥ praṇastasmādeta ṛṣayaḥ śukla iṣṭaṃ kurvantītara itarasmin || 12 ||

12. The month is the lord of creation; its dark half is, indeed, the food; the light half, the prana (eater). Therefore, the seers perform sacrifices in the light half; the others, in the other, i.e., the dark half.

Shankara’s Commentary:

Com.—The lord of creation, i.e., the year, in which this universe inheres, ends in its component part, the month. The month, indeed, the lord of creation above described, is also in the nature of a pair; of the lord of creation, i.e., the month, one portion, i.e., the dark half, is food, i.e., the moon. The other part, i.e., the light half, is the sun, the eater, the fire. Because, they see everything as prana, marked by the light half; therefore, these seers who see the prana, though performing sacrifices in the dark half, perform them, only in the light half, as they do not see the dark half, as distinct from prana (the light half). But others see not the prana and only see the unseeing dark half. Those others perform sacrifices, only in the dark half, though they do them in the light half.