Amāvasyā-Pitṛtarpaṇa: Purūravas and the Soma-Based Ancestral Offering (अमावस्या-पितृतर्पण / सोमतर्पण-विधि)
अमावसेतामृक्षे तु यदा चन्द्रदिवाकरौ / राका पञ्चदशी रात्रिरमावास्या ततः स्मृता
amāvasetāmṛkṣe tu yadā candradivākarau / rākā pañcadaśī rātriramāvāsyā tataḥ smṛtā
In the Amāvasyā nakṣatra, when Candra and Divākara—the Moon and the Sun—come together, then after the fifteenth night called Rākā, that tithi is remembered as Amāvasyā.