Matsya Purana — The Procedure and Merit of the Śubha-Saptamī Vrata
ब्रह्महत्यासहस्रस्य भ्रूणहत्याशतस्य च नाशायालमियं पुण्या पठ्यते शुभसप्तमी //
brahmahatyāsahasrasya bhrūṇahatyāśatasya ca nāśāyālamiyaṃ puṇyā paṭhyate śubhasaptamī //
This holy Śubha-Saptamī, when recited, is fully sufficient to destroy the sin of a thousand brahmin-slaughters and a hundred killings of embryos.
This verse does not discuss pralaya; it focuses on ritual merit (vrata/recitation) as a means of sin-destruction (prāyaścitta) within dharma teaching.
It presents a dharmic remedy: a householder (and even a ruler) is taught that disciplined observance/recitation on Śubha-Saptamī is a powerful act of purification, reinforcing responsibility for ethical conduct and corrective rites after wrongdoing.
The significance is ritual rather than architectural: the text praises the recitation/observance of Śubha-Saptamī as an exceptionally potent purifier, framed as a vrata-mahātmya (results of a sacred day).