Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.20

3.4.4.20

tāścaturviṃśatiḥ sampadyante | caturviṃśatirvai saṃvatsarasyārdhamāsāḥ saṃvatsarasyaivaitadrūpaṃ kriyate saṃvatsaramevaitatsaṃskaroti

These amount to twenty-four;--there being twenty-four half-moons in the year, it is thereby made of the form of the year: he thus makes up the year.