Haircuts in Halacha

Best Time for a Haircut
Before Shabbat and Holidays: Getting a haircut before Shabbat and Jewish holidays is a mitzvah to honor these special days. Ideally, do it on Friday, or Thursday if needed. The closer to Shabbat or the holiday, the better
Basic of Kosher
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Shabbat and Holidays: Haircuts are prohibited as cutting hair is one of the 39 forbidden types of work (gozez).
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Chol Hamoed (Intermediate Days of Sukkot and Passover): Haircuts are generally not allowed to ensure people look neat before the holiday begins. Exceptions include being in mourning, traveling, or other unavoidable circumstances.
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Rosh Chodesh: Haircuts are avoided for mystical reasons. Some permit haircuts if Rosh Chodesh falls on a Friday, but others, like Chabad, do not.
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The Omer Period (Between Passover and Shavuot): Haircuts are usually avoided as a sign of mourning. Customs vary on which days this applies.
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The Three Weeks (17th of Tammuz to Tisha B'Av): Haircuts are avoided as a sign of mourning. Sephardic customs may allow haircuts until the week of Tisha B'Av.
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Mourning (Avelut): Haircuts are avoided for 30 days after losing a close relative and up to 12 months after losing a parent, though restrictions may be lifted earlier.
Basic of Kosher
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Prohibition: The Torah commands not to "round off" the hair at the edges of the head, referring to the sideburns (payot) between the ears and temples.
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Length: The payot should not be cut too short. The hair should be at least long enough to fold over and touch its root, roughly 0.5 centimeters (about a number 2 clipper cut).
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Custom: Boys typically get their first haircut, called upsherin, at age three, leaving the payot uncut.