Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.24

6.2.2.24

yadvevāṣṭakāyām | parvaitatsavatsarasya yadaṣṭakā parvaitadagneryadukhā parvaṇyeva tatparva karoti

And as to why (it is performed) on the eighth day;--that eighth day no doubt is a joint of the year, and that fire-pan is a joint of Agni (the fire-altar): he thus makes joint upon joint.