Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.3.2

5.5.3.2

saṃvatsaraṃ na vapate | saṃvatsarasammitā vai vratacaryā tasmātsaṃvatsaraṃ na vapate sa eṣa vratavisarjanīyopayogo nāma stomo bhavati keśavapanīyaḥ

He does not shave his hair for a year,--religious observance is of equal measure with the year, hence he does not shave for a year: the Kesavapanîya, namely, is a (day of) praise-singing (stoma) with the view of the termination of the religious performance.