Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
मन्वादयो मानवानां श्राद्धेष्वत्यंतपुण्यदा ।भाद्रे कृष्णत्रयोदश्यां मघामिंदुः करे रविः ॥ १५२ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 152.
These are the manvadi (first day of a Manu age) days for men. They give great merit in shraddha (ancestor rite). On the dark 13th in Bhadrapada, the Moon in Magha and the Sun in Hasta ...
Simple meaning: These fourteen days are very good for ancestor rites. A dark 13th in Bhadrapada with Moon in Magha is special.
गजच्छाया तदा ज्ञेया श्राद्धे ह्यत्यंतपुण्यदा ।
That day is Gajachchhaya, very good for ancestor rites. Three moon days on one weekday mean a lost moon day.
तिथिर्वारत्रये त्वेका ह्यधिका द्वे च निंदिते ।
One moon day over three weekdays is an extra moon day. Both the lost and the extra are condemned.
विद्यते सा त्वखंडा स्यान्यूना चेत्खंडसंज्ञिता ।
A moon day lasting to sunset is whole, a shorter one partial. Each moon day splits into fifteen named parts.