The Jewel-Garland of Astrology

...in [secular] actions. The Civil monthoriginal: "Savana"; a month of exactly 30 days, used for administrative purposes and calculating the duration of specific religious vows. is considered [prescribed] in all vows and similar rituals. ॥ 21 ॥

Madhu, Madhava, Shukra, and Shuchi; Nabhas and Nabhasya;
likewise Isha, Urja, Sahas, and Sahasya; Tapas
and Tapasya—these are the [Vedic] names [of the twelve months] in their proper sequence. These archaic Vedic names correspond to the pairs of months within the six seasons: Spring (Madhu, Madhava), Summer (Shukra, Shuchi), Monsoon (Nabhas, Nabhasya), Autumn (Isha, Urja), Winter (Sahas, Sahasya), and the Dewy Season (Tapas, Tapasya). ॥ 22 ॥

The months share the names of their presiding constellations original: "Nakshatras"; the months are named based on which star the full moon occurs near, such as Chaitra being named after the star Chitra..
The two fortnights are assigned to the Gods and the Ancestors respectively.
Indeed, the Bright and Dark fortnights are established;
they are praised for [the performance of] auspicious and inauspicious actions.
॥ 23 ॥

॥ Thus ends the Chapter on the Year in the Gems of Astrology composed by the venerable teacher Shripati Bhatta. ॥ Now [begins the next section]. ॥ 1 ॥

The five [types of] lunar days: the Creator Brahma, the Destroyer Shiva, the Remover of Obstacles Ganesha, the Lord of Death Yama, and the Cold-rayed One
The Moon