Matsya Purana — Soma
दिवा पर्व त्वमावास्यां क्षीणेन्दौ धवले तु वै तस्माद्दिवा त्वमावास्यां गृह्यते यो दिवाकरः //
divā parva tvamāvāsyāṃ kṣīṇendau dhavale tu vai tasmāddivā tvamāvāsyāṃ gṛhyate yo divākaraḥ //
The Parva (sacred junction-day) of Amāvāsyā is to be observed in the daytime, when the Moon has waned away, even if the sky appears bright. Therefore, on Amāvāsyā the Parva is to be taken as occurring by day, marked by the presence of the Sun.
This verse is not about Pralaya; it focuses on dharma-based timekeeping—how to determine the Amāvāsyā parva for ritual observance.
It guides correct ritual timing: a king or householder should follow proper tithi/parva determination for offerings, śrāddha-related acts, or Amāvāsyā observances, taking the parva as daytime when the Moon is fully waned.
Ritually, it is a tithi-nirṇaya rule: Amāvāsyā’s parva is accepted in daytime (signaled by the Sun) when the Moon is depleted—useful for scheduling rites and auspicious/required timings.