Matsya Purana — Sādhāraṇa Śrāddha: General Ancestral Rite
श्रावणस्याष्टमी कृष्णा तथाषाढी च पूर्णिमा कार्त्तिकी फाल्गुनी चैत्री ज्येष्ठपञ्चदशी सिता मन्वन्तरादयश् चैता दत्तस्याक्षय्यकारिकाः //
śrāvaṇasyāṣṭamī kṛṣṇā tathāṣāḍhī ca pūrṇimā kārttikī phālgunī caitrī jyeṣṭhapañcadaśī sitā manvantarādayaś caitā dattasyākṣayyakārikāḥ //
The dark eighth (Aṣṭamī) of Śrāvaṇa; the full-moon day of Āṣāḍha; likewise the full-moon days of Kārttika, Phālguna, and Caitra; the bright fifteenth of Jyeṣṭha; and Manvantara and similar sacred occasions—gifts given at these times become causes of imperishable (akṣaya) merit.
It does not discuss Pralaya directly; it frames Manvantara-related occasions as sacred times when charitable acts yield imperishable (akṣaya) spiritual results.
It guides householders and rulers in dāna-dharma: giving charity on specified lunar dates and sacred periods is presented as a disciplined duty that multiplies merit and supports social-religious order.
The significance is ritual-calendar based (tithi/occasion selection): it lists specific dates—Aṣṭamī and various Pūrṇimās—on which gifts and rites are considered especially efficacious, producing akṣaya merit.