Matsya Purana — Ekoddiṣṭa Śrāddha
सदैव पितृहा स स्यान् मातृभ्रातृविनाशकः मृताहे पार्वणं कुर्वन् नधो ऽधो याति मानवः //
sadaiva pitṛhā sa syān mātṛbhrātṛvināśakaḥ mṛtāhe pārvaṇaṃ kurvan nadho 'dho yāti mānavaḥ //
A man who performs the pārvaṇa-śrāddha on the very day of death becomes, as it were, a constant slayer of his forefathers and a destroyer of his mother and brothers; he goes ever further downward into lower states of suffering.
This verse is not about pralaya; it teaches ritual-ethical causality (karma) by warning that mis-timed ancestral rites lead to spiritual downfall.
It instructs householders (and kings as guardians of dharma) to follow correct śrāddha timing; performing a lunar-parvan śrāddha on an inauspicious death-related day is treated as a grievous fault with severe karmic results.
The significance is ritual: it specifically cautions against performing pārvaṇa-śrāddha on a mṛtāha (death-day), emphasizing proper calendrical eligibility (kāla-śuddhi) for ancestral offerings.