Matsya Purana — Soma
तस्मात्तु पर्वणो ह्यादौ प्रतिपद्यादिसंधिषु सायाह्ने अनुमत्याश्च द्वौ लवौ काल उच्यते लवौ द्वावेव राकायाः कालो ज्ञेयो ऽपराह्णिकः //
tasmāttu parvaṇo hyādau pratipadyādisaṃdhiṣu sāyāhne anumatyāśca dvau lavau kāla ucyate lavau dvāveva rākāyāḥ kālo jñeyo 'parāhṇikaḥ //
Therefore, at the beginning of a parvan (festival-junction)—at junctions such as Pratipadā and the like—the parvan-time is said to be two lavas in the evening; and likewise, for Rākā (the full-moon night), the time should be known as two lavas in the afternoon (aparāhṇa).
Nothing directly—this verse is technical dharma/kalā-nirṇaya, defining precise parvan timing around lunar-day junctions rather than cosmology or pralaya.
It supports correct observance of vratas and festival rites: a king (as patron of public dharma) and a householder (as performer of domestic rites) must time parvan worship/fasting accurately, especially at tithi junctions like Pratipadā and at Pūrṇimā (Rākā).
Ritual significance: it specifies the operative window (two lavas) for parvan observance—placed in the evening for Pratipadā-type junctions and in the afternoon for Rākā/Pūrṇimā—useful for scheduling pūjā, dāna, and vrata procedures.