Moon vocabulary · one idea
Pakṣa: the Moon’s two wings
Pakṣa can mean a side, a half, or the wing of a bird. A lunar month—māsa in Sanskrit—has two pakṣas: one while the Moon appears to grow brighter and one while it appears to grow darker.
Pakṣa · wing, side, lunar half, or lunar fortnight
Śukla grows brighter; kṛṣṇa grows darker.
Śukla pakṣa · bright wing
The waxing half, from New Moon toward Full Moon.
Kṛṣṇa pakṣa · dark wing
The waning half, from Full Moon toward New Moon.
In calendar English, a pakṣa is often called a lunar fortnight. Here that means fifteen tithis whose span in civil time varies. Once you know which wing is active, a name such as Śukla Caturthī or Kṛṣṇa Caturthī becomes much easier to place.