The month, verily, is Prajāpati. Its dark fortnight is Rayi; its bright fortnight is Prāṇa. Therefore these ṛṣis perform sacrifice in the bright fortnight; the others, in the other fortnight.

The prāṇa-rayi duality is projected onto the lunar month. Kṛṣṇapakṣa (the dark fortnight, waning) is Rayi — matter, receptivity, nutrition. Śuklapakṣa (the bright fortnight, waxing) is Prāṇa — spirit, activity, light.

This division has practical implications in Vedic tradition. Sacrifices for the ancestors (pitṛ-kārya) are performed in the dark fortnight, while sacrifices for the gods (deva-kārya) are performed in the bright fortnight.

For the yogī, this is a metaphor for cycles of activity and rest, of inhalation and exhalation. Prāṇa flows in waves — there are moments of expansion (prāṇa predominant) and moments of contraction (rayi predominant).

In haṭha yoga, certain practices are recommended according to lunar phase. The bright fortnight is favorable for expansive practices, while the dark fortnight is propitious for introverted practices and consolidation.

The underlying teaching: everything in manifestation operates according to cycles of complementary opposites. Knowing these cycles allows one to harmonize with cosmic forces.