Matsya Purana — Soma
यस्माद् आपूर्यते सोमः पञ्चदश्यां तु पूर्णिमा दशभिः पञ्चभिश्चैव कलाभिर्दिवसक्रमात् //
yasmād āpūryate somaḥ pañcadaśyāṃ tu pūrṇimā daśabhiḥ pañcabhiścaiva kalābhirdivasakramāt //
Because Soma (the Moon) becomes filled up, the fifteenth lunar day is called the Full Moon (Pūrṇimā); day by day, in due sequence, it is completed by fifteen kalās (lunar digits).
It does not describe Pralaya directly; it teaches cosmic order through lunar time—how the Moon’s fifteen kalās culminate in Pūrṇimā, reflecting regulated cosmology rather than dissolution.
By defining Pūrṇimā via the Moon’s completed kalās, it supports correct calendrical observance—important for kings and householders to schedule vows, gifts, and rites on properly determined tithis.
The ritual takeaway is timing: Pūrṇimā is identified as the fifteenth tithi when Soma is ‘full’ with fifteen kalās, a key marker for full-moon observances and offerings.