Matsya Purana — The Attendant Hosts of the Sun and Moon: Monthly Gaṇas
ततः पञ्चदशे भागे किंचिच्छेषे निशाकरे ततो ऽपराह्णे पितरो जघन्यदिवसे पुनः //
tataḥ pañcadaśe bhāge kiṃciccheṣe niśākare tato 'parāhṇe pitaro jaghanyadivase punaḥ //
Then, when the Moon has only a slight remainder in its fifteenth portion (that is, when the phase is nearly complete), and thereafter again in the afternoon on the final day, the Pitṛs (ancestral spirits) are to be invoked and receive the offerings once more.
This verse is not about Pralaya; it gives a ritual timing rule for Pitṛ-offerings, specifying auspicious windows tied to the lunar division and the afternoon period.
It supports the householder’s dharma of honoring ancestors through properly timed Śrāddha; for rulers too, correct public adherence to such rites is part of sustaining social-religious order (dharma).
The significance is ritual (not architectural): it prescribes the proper time—near the completion of the lunar phase and again in aparāhṇa on the final day—for inviting the Pitṛs and offering Śrāddha.