Shatapatha Brahmana 6.6.1.5

6.6.1.5

sa ya eṣa āgnāvaiṣṇavaḥ | tasya tadeva brāhmaṇaṃ yatpuraścaraṇe vaiśvānaro dvādaśakapālo vaiśvānaro vai sarve 'gnayaḥ sarveṣāmagnīnāmupāptyai dvādaśakapālo dvādaśa māsāḥ saṃvatsaraḥ saṃvatsaro vaiśvānaraḥ

Now as regards that (cake) for Agni and Vishnu, its mystic import is the same as what is (implied) in a preparatory ceremony. And the (cake) on twelve potsherds for Vaisvânara is for the obtainment of all the fires, Vaisvânara being all the fires;--it is one of twelve potsherds, for twelve months are a year, and Vaisvânara is the year.