Among the saṁskāras or sacraments enumerated by the ancient writers on the dharmaśāstras, like Gautama, four vedavratas are important (vide Gautama Dharmasūtras 8.15). They are: mahā-nāmnī-vrata, mahā-vrata, upaniṣad-vrata (or aupa-niṣada-vrata) and godāna-vrata. Each of these vratas had to be observed by the brahmacārin (celibate student of the Vedas) for a year each.
Other works like the Śāṅkhāyana Gṛhyasūtras (2.11-12) describe these four vratas as śukriya, śākvara, vrātika and aupaniṣada. They are the vratas that precede the study of the different sections of the Vedas.
The aupaniṣada-vrata precedes the study of the Upaniṣad part of the Vedas.
At the beginning of each of these vratas there has to be a separate upa-nayana ceremony involving the wearing of the new yajñopavīta (sacred thread), new deer-skin and new girdle. The vrata lasts for one year.
Failure to observe these vratas or lapses during their performance had to be expiated with appropriate prāyaścittas specially prescribed. The observance of these vratas seems to have been gradually given up.