This Halacha is an excerpt from our Sefer
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On the one hand, it is a Mitzvah to get a haircut on Erev Shavuos in honor of Yom Tov, in order so one does not enter into the holiday looking unpresentable. On the other hand, however, even if it is past one’s accustomed mourning period, according to Kabbalah one is to avoid getting a haircut throughout the period of Sefira, including during the Shloshes Yimei Hagbala, until Erev Shavuos, and so is the Chabad custom.
Accordingly, in years that Erev Shavuos falls on Shabbos we enter into a dilemma of what to do; should we not get a haircut on Erev Shabbos due to the Kabalistic warning, or should we ignore the Kabalistic warning and get a haircut due to the Mitzvah involved in doing so?
B. The law:
When Erev Shavuos coincides with Shabbos, one may get a haircut on Erev Shabbos which is the 48th day of the Omer. This applies even according to the Kabbalistic ruling and Chabad custom. [One may begin to take a haircut after sunrise of Erev Shabbos, although one should not do so the night before.] However, when Shavuos falls on Monday, one is not to get a haircut on Erev Shabbos, but only on Erev Shavuos.
One is to get a haircut on Erev Shabbos, after sunrise.
Other opinions: Some write that according to even according to Kabbalah one may get a haircut on the 48th day of Omer and that so was the custom of Rav Chaim Vital. [Minchas Elazar ibid]
Other opinions: The Rashash questioned whether one may get a haircut on the 48th day of Omer even in such a case. He concluded that if he would not have been forced into it, he would not have gotten a haircut on the 48th. [Nehar Shalom p. 25]
Other opinions: Some Poskim rule one may get a haircut on the night before Shavuos even according to the Kabalistic ruling. [Pekudas Eliezer 493:2 that so is the custom, and so was the custom of Rav Chaim Vital to get a haircut on the night of the 49th of Omer, after saying Sefiras Haomer] Perhaps, according to this opinion, one may also get a haircut on the night of the 48th in years that Shavuos falls on Motzei Shabbos.