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Getting a haircut on Erev Rosh Hashanah:
It is a Mitzvah upon a person to get a haircut on Erev Rosh Hashanah in honor of Yom Tov, in order so one does not enter into the holiday looking not presentable.
When may one to get a haircut on Erev Rosh Hashanah?
It is permitted to get a haircut throughout the entire Erev R”H, 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 that 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.
 Michaber 581/4; Admur 529/2 regarding every Erev Yom Tov; 529/5 that this Mitzvah applies likewise on [Erev] Rosh Hashanah.
 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.
 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.