Narada Purana — Purva Bhaga, Shloka 48
Tithi-Nirṇaya for Vratas: Ekādaśī Rules, Saṅkrānti Punya-kāla, Eclipse Observances, and Prāyaścitta
एकादशी द्वादशी च रात्रघिशेषे त्रयोदशी । द्वादशद्वादशीपुण्यं त्रयोदश्यां तु पारणे ॥ ४८ ॥
ekādaśī dvādaśī ca rātraghiśeṣe trayodaśī | dvādaśadvādaśīpuṇyaṃ trayodaśyāṃ tu pāraṇe || 48 ||
若持斋之行跨越十一斋日（Ekādaśī）与十二日（Dvādaśī），而十二日仅余夜分些许，以致十三日（Trayodaśī）几如现前，则能得如法十二日之功德——但须于十三日行破斋（pāraṇa）。
Sanatkumara (teaching Narada)
Vrata: Ekādaśī-vrata
Primary Rasa: shanta
Secondary Rasa: bhakti
FAQs
It defines a precise dharma rule: when tithi-boundary conditions make Dvādaśī effectively lapse into Trayodaśī, one should still preserve the sanctity of the observance by performing pāraṇa on Trayodaśī, thereby retaining Dvādaśī’s merit.
By emphasizing correct pāraṇa timing for Ekādaśī-related vows, it shows bhakti as disciplined worship—devotion expressed through careful observance of Viṣṇu-oriented vrata rules rather than mere sentiment.
Kalā/gaṇita-based tithi discernment (calendar computation) and ritual timing—applied knowledge akin to Vedāṅga Jyotiṣa used to decide when pāraṇa should be done under tithi overlap.