Matsyapurana 14.8

14.8

dhairyeṇa tasya sā lokair amāvāsyeti viśrutā
pitṝṇāṃ vallabhā tasmāt tasyāmakṣayakārakam //

She used to live in heaven, owing to her supernatural powers; but the moment she had such a base desire she fell to the earth. The date on which she expressed such a desire and Amâvasu Pitṛi did not give way to passion, came to be known as Amâvâsyâ Tithi. Amâvâsyâ is, consequently, very dear to the Pitṛis, and offerings made to them on that Tithi are very heartily accepted by them.—7-8.