Amāvasyā-Pitṛtarpaṇa: Purūravas and the Soma-Based Ancestral Offering (अमावस्या-पितृतर्पण / सोमतर्पण-विधि)
दशभिः पञ्चभिश्चैव कलाभिर्दिवसक्रमात् / तस्मात्कलाः पञ्चदश सोमेनास्य तु षोडशी
daśabhiḥ pañcabhiścaiva kalābhirdivasakramāt / tasmātkalāḥ pañcadaśa somenāsya tu ṣoḍaśī
In the succession of days, (the moon) waxes by ten and by five kalās; therefore Soma’s kalās are fifteen, and yet there is also his sixteenth, called Ṣoḍaśī—the mark of completeness.