Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.17

3.4.4.17

sa vai tisra upasada upeyāt | trayo vā ṛtavaḥ saṃvatsarasya saṃvatsarasyaivaitadrūpaṃ kriyate saṃvatsaramevaitatsaṃskaroti dvirekayā pracarati dvirekayā

Let him undertake three Upasads; for, there being three seasons in the year, it is thereby made of the form of the year: he thus makes up the year. He performs twice each.