Matsya Purana — Soma
पूर्णेन्दुः पूर्णपक्षे तु रात्रिसंधिषु पूर्णिमा तस्मादाप्यायते नक्तं पौर्णमास्यां निशाकरः //
pūrṇenduḥ pūrṇapakṣe tu rātrisaṃdhiṣu pūrṇimā tasmādāpyāyate naktaṃ paurṇamāsyāṃ niśākaraḥ //
In the bright fortnight (śukla-pakṣa), the night of Pūrṇimā is the junction of the night; therefore, on Paurṇamāsī, the Moon—lord of the night—waxing by night, becomes full.
This verse is not about Pralaya; it explains lunar waxing and the ritual/astronomical significance of Pūrṇimā as a night-junction (rātri-saṃdhi) in the bright fortnight.
By defining Paurṇamāsī and its night-junction character, it supports correct timing for household and royal observances—such as Pūrṇimā-vrata, dāna, and night worship—where dharma depends on accurate tithi and sandhi determination.
Ritually, it highlights Paurṇamāsī night as especially potent for worship and vows because the Moon is said to ‘fill’ at night; it does not give Vāstu or temple-construction rules in this specific verse.