Matsya Purana — Soma
अमा वसेतामृक्षे तु यदा चन्द्रदिवाकरौ एका पञ्चदशी रात्रिर् अमावास्या ततः स्मृता //
amā vasetāmṛkṣe tu yadā candradivākarau ekā pañcadaśī rātrir amāvāsyā tataḥ smṛtā //
When the Moon and the Sun are together in the same lunar mansion (nakṣatra), that single fifteenth night is remembered as Amāvāsyā, the new-moon night.
This verse does not discuss Pralaya; it defines Amāvāsyā in calendrical terms—useful for fixing sacred time rather than describing cosmic dissolution.
By precisely defining Amāvāsyā, the text supports correct scheduling of household rites (e.g., pitṛ-kārya/śrāddha, vrata, dāna) and royal/public ritual observances that depend on accurate tithi reckoning.
The ritual significance is timing: Amāvāsyā is identified as the fifteenth night when Sun and Moon conjoin in the same nakṣatra—critical for choosing auspicious/required dates for offerings, fasts, and ancestral rites.