Amāvasyā-Pitṛtarpaṇa: Purūravas and the Soma-Based Ancestral Offering (अमावस्या-पितृतर्पण / सोमतर्पण-विधि)
अन्वाधानक्रिया यस्मात्क्रियते पर्वसंधिषु / तस्मात्तु पर्वणामादौ प्रतिपत्सर्वसंधिषु
anvādhānakriyā yasmātkriyate parvasaṃdhiṣu / tasmāttu parvaṇāmādau pratipatsarvasaṃdhiṣu
Since the rite of anvādhāna is performed at the junctions of the parvas, therefore at every junction the beginning of a parva is marked by Pratipadā (the first lunar day).