Matsya Purana — Soma
यदा चन्द्रश्च सूर्यश्च नक्षत्राणां समागतौ अमावास्यां निवसत एकस्मिन्नथ मण्डले //
yadā candraśca sūryaśca nakṣatrāṇāṃ samāgatau amāvāsyāṃ nivasata ekasminnatha maṇḍale //
When the Moon and the Sun come together in relation to the nakṣatras (lunar mansions), then on the night of Amāvāsyā (new moon) they are said to dwell within a single sphere, that is, in the same celestial circle.
Directly, it does not describe Pralaya; it defines the astronomical condition of Amāvāsyā as the Moon and Sun occupying a single celestial circle, a calendrical marker often used to time rites in wider Purāṇic contexts.
It supports dharmic timekeeping: kings and householders schedule śrāddha, darśa rites, fasts, and other observances by correctly identifying Amāvāsyā—when Sun and Moon are in conjunction—so ritual acts align with prescribed tithi-nakṣatra timing.
Ritually, it indicates the precise Amāvāsyā configuration used for choosing vrata and śrāddha timings; by extension, such tithi determination is also applied in selecting auspicious days for temple consecrations and Vāstu-related ceremonies.