The custom among Ashkenazim is to refrain from shaving and haircutting throughout the three-week period of Ben Ha'mesarim, from Shiba Asar Be'Tammuz through Tisha B'Ab. Sepharadim, by contrast, observe this practice only during the week of Tisha B'Ab, from the Shabbat immediately preceding Tisha B'Ab until the 10 th of Ab. If a Sephardic boy is studying in an Ashkenazic yeshiva, where the majority of the students are refraining from shaving and haircutting already from Shiba Asar Be'Tammuz, he is nevertheless allowed to shave and take a haircut, until the week of Tisha B'Ab. However, it is preferable that he avoid doing so in order not to stand out. Hacham Ovadia Yosef writes about one who is stringent in this regard, "Tabo Alav Beracha" – he is worthy of special blessing. If a Sephardic student in an Ashkenazic institution has been accustomed to acting stringently, refraining from shaving and haircutting already from Shiba Asar Be'Tammuz, and now wishes to change his practice, he may perform Hatarat Nedarim (the annulment of vows). His acceptance of this practice constitutes a "vow," assuming he never explicitly said that he was taking on this practice "Beli Neder" – without making a formal vow. Thus, if he wishes to change his practice so he can shave or take a haircut during the Three Weeks (until the week of Tisha B'Ab), he should conduct Hatarat Nedarim in front of a Bet Din to have his commitment annulled. However, Hatarat Nedarim is required only if the student knew that this stringency was not strictly required by Halacha, and he had followed this practice as an additional measure of piety. If, however, he mistakenly thought that he must refrain from shaving and haircutting already from Shiba Asar Be'Tammuz, and he then learned that this is not required, then he may drop this stringent practice even without Hatarat Nedarim. Since this "vow" was taken on a faulty premise, as he erroneously assumed that Halacha requires him to refrain from shaving and haircutting, the "vow" is not binding, and he may therefore change his practice even without annulling his vow. Summary: Sephardic practice is to refrain from shaving and haircutting during the week of Tisha B'Ab. A Sephardic student studying in an institution where the majority of students are Ashkenazim – who refrain from shaving and haircutting already from Shiba Asar Be'Tammuz – may shave and take a haircut until the week of Tisha B'Ab, even though the other students refrain from doing so. Nevertheless, it is preferable him to refrain in deference to the Ashkenazic students. If he had observed this stringency and now wishes to act leniently, he may perform Hatarat Nedarim. If he had observed this stringency because he mistakenly thought it was Halachically required, then he does not need Hatarat Nedarim.