Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 1 · Muhurta: Marriage, Home, Farming
ज्योतिःशास्त्रम्
कृष्णाष्टमीदिनादूर्ध्वं यावच्छुक्लाष्टमीदिनम् ।तावत्कालं शशी क्षीणः पूर्णस्तत्रोपरि स्मृतः ॥ ६२ ॥
In the full Agni Purana this is chapter 121, verse 62.
From the dark 8th day to the bright 8th day, the Moon is weak. After that, it is called full.
Simple meaning: The Moon is weak from the dark 8th to the bright 8th. After that, it is full.
वृषे च मिथुने भानौ जीवे चन्द्रेन्द्रदैवते ।
A Thursday full moon with Jupiter and the Moon in Jyeshtha is called Mahajyaishthi.
ऐन्द्रे गुरुः शशी चैव प्राजापत्ये रविस्तथा ।
Jupiter and the Moon in Jyeshtha and the Sun in Rohini make the full moon Mahajyaishthi.
स्वात्यन्तरे यन्त्रनिष्ठे शक्रस्योत्थापयेद्ध्वजम् ।
Indra's flag is raised at a set time and taken down after a week.