Matsya Purana — Soma
यदान्योन्यवतीं पाते पूर्णिमां प्रेक्षते दिवा चन्द्रादित्यो ऽपराह्णे तु पूर्णत्वात्पूर्णिमा स्मृता //
yadānyonyavatīṃ pāte pūrṇimāṃ prekṣate divā candrādityo 'parāhṇe tu pūrṇatvātpūrṇimā smṛtā //
When, in the daytime, the Moon and the Sun are seen facing one another in opposition, then—if this occurs in the afternoon (aparāhṇa)—because the lunar digit (kalā) has become full, that day is remembered as Pūrṇimā, the Full-Moon day.
This verse does not discuss Pralaya; it gives a calendrical (kāla-nirṇaya) rule for identifying Pūrṇimā based on the observable relationship of the Sun and Moon.
By defining Pūrṇimā precisely, it supports correct timing for vows, fasting, śrāddha, and public or royal rituals—duties that householders and kings must perform according to proper lunar dates.
Its ritual significance is time-selection: it helps decide the correct Pūrṇimā for performing pūjā, vratas, and other observances that require accurate tithi determination (useful alongside Matsya Purana’s broader ritual and temple-practice guidance).