Matsya Purana — Sādhāraṇa Śrāddha: General Ancestral Rite
यस्यां मन्वन्तरस्यादौ रथमास्ते दिवाकरः माघमासस्य सप्तम्यां सा तु स्याद्रथसप्तमी //
yasyāṃ manvantarasyādau rathamāste divākaraḥ māghamāsasya saptamyāṃ sā tu syādrathasaptamī //
That seventh lunar day (saptamī) of the month of Māgha—on which, at the very beginning of a Manvantara, the Sun (Divākara) is said to mount his chariot—is indeed known as Ratha-saptamī.
It situates ritual time within cosmic time: the day called Ratha-saptamī is linked to the commencement of a Manvantara, emphasizing how Puranic calendrics mirror large-scale cosmic cycles rather than directly describing Pralaya here.
It provides a dharma-calendar marker: kings and householders are expected to observe auspicious tithis for worship—here, Māgha saptamī for Sūrya—supporting righteous conduct through regular vrata and public/household ritual order.
The ritual significance is primary: Ratha-saptamī is defined as a Sun-related observance day (Māgha saptamī) tied to the Sun’s chariot symbolism, commonly prompting Sūrya worship and related rites rather than Vāstu rules in this specific verse.