Agniveshyagrihyasutra 2.4.4.3

2.4.4.3

amāvāsyāyāṃ paurṇamāsyāṃ vā keśaśmaśrulomanakhāni vāpayitvā snātaḥ śuddhavāsā brahmacārikalpena vratamupaiti vatsaraṃ māsaṃ caturviṃ śatimahorātraṃ dvādaśarātraṃ ṣaḍrātraṃ trirātraṃ vā \

On the new moon or on the full moon, having had hair, beard, body-hair, and nails shaved, bathed, wearing pure garments, he undertakes a vow in the manner of a Vedic student for a year, a month, twenty-four days and nights, twelve nights, six nights, or three nights.