Matsyapurana 141.41

141.41

atyarthaṃ rājate yasmāt paurṇamāsyāṃ niśākaraḥ
rañjanāccaiva candrasya rāketi kavayo viduḥ //

On the completion of the phases, Pûrṇimâ occurs during the periodical junction of the Moon in the night when the Moon is full, hence, that night is called the full moon night when the Moon is greatly pleased. When, by the mutual opposition of the Sun and the Moon, the Pûrṇimâ takes place in the afternoon, then, the evening is said to be the Pûrṇimâ when the Moon shines with all His phases completed by the Sun. The Devas and the Pitṛis adore Him (the Moon), therefore He is called Anumati, and, on account of the full moon it is called Pûrṇimâ. The Moon is highly luminous on the night of Pûrṇimâ and therefore, He is called Râkâ.—38-41.