Matsya Purana — The Rohiṇī–Candraśayana Vow
यदा सोमदिने शुक्ला भवेत्पञ्चदशी क्वचित् अथवा ब्रह्मनक्षत्रं पौर्णमास्यां प्रजायते //
yadā somadine śuklā bhavetpañcadaśī kvacit athavā brahmanakṣatraṃ paurṇamāsyāṃ prajāyate //
Whenever, on a Monday, the fifteenth lunar day of the bright fortnight (the full-moon tithi) occurs, or whenever the Brahma-nakṣatra arises on the full-moon day, this is regarded as an especially significant and auspicious occurrence.
This verse does not discuss Pralaya; it focuses on auspicious calendrical timing—tithi and nakshatra combinations connected with Paurṇimā observances.
It guides householders (and patrons such as kings) in choosing highly auspicious dates—Monday Paurṇimā or Paurṇimā with Brahma-nakṣatra—for conducting vows, gifts, and worship with enhanced merit.
The significance is ritual (not architectural): it highlights a special yoga of weekday–tithi and nakshatra on Paurṇimā, implying an especially favorable time for vrata, pūjā, dāna, and related observances.