Matsya Purana — The Ārdrānandakarī Tṛtīyā Vrata: Ritual Procedure
यदा शुक्लतृतीयायाम् आषाढर्क्षं भवेत्क्वचित् ब्रह्मर्क्षं वा मृगर्क्षं वा हस्तो मूलमथापि वा दर्भगन्धोदकैः स्नानं तदा सम्यक्समाचरेत् //
yadā śuklatṛtīyāyām āṣāḍharkṣaṃ bhavetkvacit brahmarkṣaṃ vā mṛgarkṣaṃ vā hasto mūlamathāpi vā darbhagandhodakaiḥ snānaṃ tadā samyaksamācaret //
Whenever, on the third lunar day of the bright fortnight (Śukla Tṛtīyā), the nakṣatra happens to be Āṣāḍhā, or Brahmā’s star Rohiṇī, or Mṛga (Mṛgaśīrṣa), or Hasta, or Mūla, then one should duly perform the ritual bath with kuśa-grass and fragrant water.
This verse does not address pralaya; it gives a dharma-oriented timing rule for purification—an auspicious bath when Śukla Tṛtīyā coincides with specific nakṣatras.
It prescribes a calendrical discipline (tithi–nakṣatra observance) for maintaining ritual purity; such practices were expected of householders and rulers alike to uphold dharma and public religious order.
The ritual significance is snāna-vidhi: bathing with darbha (kuśa grass) and fragrant water on specified lunar/stellar combinations—useful for vrata observance and preparatory purification before worship.