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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.
May one get a haircut during Shloshes Yimei Hagbala, and when Erev Shavuos coincides with Shabbos?
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.
On Erev Shavuos: It is permitted, and is a Mitzvah, to get a haircut on Erev Shavuos. One may do so even past midday.
Erev Shavuos that falls on Shabbos: 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 not the night before.] However, when Shavuos falls on Monday, one is not to get a haircut on Erev Shabbos, but only on Erev Shavuos.
May those who follow the Kabbalistic custom get a haircut on the night of Erev Shavuos [5th of Sivan]?
Some Poskim rule one may get a haircut on the night before Shavuos even according to the Kabalistic ruling. Other Poskim however rule one is to avoid getting a haircut at night until daybreak. Those who are stringent like all the Sefira customs do not get a haircut until the morning of Erev Shavuos.
When may one get a haircut on Erev Shavuos?
It is permitted to get a haircut throughout the entire Erev Shavuos, even past the time of Mincha. This applies even to a professional haircut that is being done by a Jew in exchange for payment.
Midday: Some Poskim rule that [despite the above ruling] one is to beware to take a haircut prior to midday. Others however rule that doing so is not required, and so is implied to be the opinion of Admur. Furthermore some rule one is to try to have a haircut specifically past midday so it be recognizable he is doing so in honor of Yom Tov. Even according to the above stringent opinion, if one did not take a haircut prior to midday he may do so afterwards, although there are some who are stringent in such a case not to take a haircut.
May one get a haircut prior to Davening?
The Kavanos of a Haircut:
One is to intend upon getting a haircut that he is removing the powers of severity and is fulfilling the Mitzvah of having Peyos and the Mitzvah of paying a worker for his job on the same day and the Mitzvah of honoring Yom Tov. One can fulfill a total of fourteen Mitzvos when he gets a haircut.
 Admur 529:2 regarding every Erev Yom Tov
 Custom of Arizal, brought in Peri Eitz Chaim 22:7; Shaar Hakavanos 86; Nehar Shalom 25; Igeres Ramaz 2; Birkeiy Yosef 493:6; Moreh Baetzba 8:221; Minchas Elazar 4:44; Kaf Hachaim 493:13
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]
 Hayom Yom p. 53; Sefer Haminhagim p. 86 [English] “The Rebbe Rashab was uneasy with those which took haircuts during the Shloshes Yimei Hagbala”
 Moreh Baetzba 8:221; Minchas Elazar 3:65; Kaf Hachaim 493:13; Shevach Hamoadim p. 235 footnote 10 in name of Rav Shmuel Levitin that so was the custom in Lubavitch; Otzer Minhagei Chabad p. 290
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]
 The reason: Although in general, based on the custom of the Arizal, we avoid haircuts until Erev Shavuos, this only applies when Erev Shavuos falls on a weekday, as one cannot enter into the festival with improper attire. Furthermore, it is recorded in Shaar Hakavanos that Rav Chaim Vital was always accustomed to get a haircut on the 48th day of Omer. Some learn this itself was due to Kabalistic reasons. Furthermore, even according to the Rashash who leaned to be stringent not to take a haircut on Erev Shabbos in such a coincidence, nevertheless he too forced himself to do so. Hence there is no room to be stringent on the above, and on the contrary one should do so. [See Minchas Elazar ibid]
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 Hagahos Chasam Sofer 493
 Pekudas Eliezer 493:2
 The reason: 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. [ibid]
 Moed Lecho Chaiy 6:7
 The reason: As night time is a time of Gevuros, and hence if we avoid a haircut during Sefira, even more so is it to be avoided at night. [ibid]
 See Admur 493:3 that according to those who start on the second of Sivan the 33rd day falls on Erev Shavuos, and Miktzas Hayom Kekulo.
 Admur 251:4 regarding Erev Shabbos and Erev Yom Tov [see 251:1 that includes Erev Yom Tov in these laws]; Mateh Efraim 625:11 “One may take a haircut throughout the entire day”; See “The Laws and Customs of Erev Shabbos” Chapter 2 for the full details of this subject!
 Custom of the Arizal brought in M”A 251:5; Gr”a brought in Aruch Hashulchan 251:3; Kitzur Shulchan Aruch 128:15; Mateh Efraim 581:50; Mateh Efraim 625:11 “It is a Mitzvah Min Hamuvchar…”; Alef Lamagen 581:108 “Is a Mitzvah Min Hamuvchar”; Kaf Hachaim 260:13; 581:80 [proper to be stringent]
 Aruch Hashulchan 251:3; Alef Lamagen 581:108 rules that the above ruling to take a haircut prior to midday is a Mitzvah Min Hamuvchar and if one did not do so beforehand then he may do so until Mincha Ketana. [Vetzaruch Iyun Gadol on the wording “Mincha Ketana” as everyone agrees a haircut may be taken the entire day.
The Magen Avraham 251:5 records that the Arizal would not get a haircut past the time of Mincha Gedola [midday], based on Kabalistic reasons. Admur omitted this custom of the Arizal in 251. See Shaareiy Halacha Uminhag 1:130 that the Rebbe concludes regarding the time of cutting nails, which is similar to the time of the cutting of the hair, that he did not receive a directive in how to follow. The Aruch Hashulchan ibid states that the Gra is stringent against allowing haircuts past midday, although this is not the worldly custom.
 Based on Aruch Hashulchan 260:6; See also 251:3
 Mateh Efraim 625:11 “It is a Mitzvah Min Hamuvchar…”; Alef Lamagen 581:108
 Mateh Efraim ibid based on the Arizal who would refrain from cutting his hair after midday.
 Admur 70:5
 Admur 89:8
 Shaar Hamitzvos brought in Kaf Hachaim 232:15; 581:80
 Kaf Hachaim 232:15: 7 negative commands relating to the Peyos and beard; One negative command of Bechukoseihem Lo Seliechu; One negative command of Lo Yilbash Gever; One negative command of Lo Savo Alav Hashemesh; One negative command of Lo Salin; One positive command of Beyomo Titen Secharo; If the barber is poor he also fulfills the negative command of Lo Yikra Alecha; One Rabbinical Mitzvah of honoring Shabbos and Yom Tov.