Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.17

6.2.2.17

tadvai paurṇamāsyāmeva | asau vai candraḥ paśustaṃ devāḥ paurṇamāsyāmālabhante yatrainaṃ devā ālabhante tadenamālabhā iti tasmātpaurṇamāsyāṃ yadveva paurṇamāsyām paurṇamāsī ha vāva prathamā vyuvāsa tasmādveva paurṇamāsyām

But, indeed, this (takes place) at full moon, for the victim is yonder moon, and him the gods slaughter at full moon: 'I will slaughter him at the time when the gods slaughter him,' thus he thinks, and therefore (he does so) at full moon. And, again, why at full moon;--the full moon no doubt was the first to shine forth, hence also (the sacrifice takes place) at full moon.