Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 1 · Muhurta: Marriage, Home, Farming
ज्योतिःशास्त्रम्
वृषे च मिथुने भानौ जीवे चन्द्रेन्द्रदैवते ।पौर्णमासी गुरोर्वारे महाज्यैष्ठी प्रकीर्तिता ॥ ६३ ॥
In the full Agni Purana this is chapter 121, verse 63.
The Sun is in Taurus or Gemini. Jupiter and the Moon are in Jyeshtha. A full moon on Thursday then is called Mahajyaishthi.
Simple meaning: 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.
सर्वं हेमसमं दानं सर्वे ब्रह्मसमा द्विजाः ।
On the day of a solar eclipse, every gift, Brahmin and water is held to be very holy.