Shatapatha Brahmana 13.5.1.7

13.5.1.7

yadvevaikaviṃśam ekaviṃśo vai stomānām pratiṣṭhā bahu khalu vā etadetasminnahanyuccāvacamiva karma kriyate tadyadetadetasminnahanyuccāvacam bahu karma kriyate tadetasminnekaviṃśe pratiṣṭhāyām pratiṣṭhitaṃ kriyātā iti tasmādvevaitadekaviṃśamahaḥ

And, again, as to why it is an ekavimsa day;--the ekavimsa, assuredly, is the foundation of Stomas, and manifold is that ever-varying performance which takes place on this day,--and it is because he thinks that that manifold and ever-varying performance which takes place on this day, shall take place so as to be established on the ekavimsa as a firm foundation, that this is an ekavimsa day.