Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
गजच्छाया तदा ज्ञेया श्राद्धे ह्यत्यंतपुण्यदा ।एकस्मिन्वासरे तिस्रस्तिथयः स्यात्तिथिक्षयः ॥ १५३ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 153.
... know it then as Gajachchhaya. It gives great merit in shraddha (ancestor rite). When three moon days fall on one weekday, a tithi (moon day) is lost.
Simple meaning: 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.
क्षणसंज्ञास्तदर्द्धानि तासामर्द्धप्रमाणतः ।
Those fifteen parts are called kshanas. The Sun is fixed, the Moon movable, Mars cruel and Mercury mixed.