Kaushitakagrihyasutra 2.7.8

2.7.8

yāṃ vānyāṃ bhapraśastāṃ manyeta tasyāṃ śukriyabrahmacaryamādiśeta \8\

or on whatever other [tithi] he regards as auspicious with regard to the asterisms, on that he should assign the Śukriya vow of studenthood.