Hi rabbi— What is the Kabbalah you referenced in the halachos of not getting a haircut the entire time?! Thank you! Y

Shalom, Thank you for your question. The custom of not cutting hair during the entire period of the Omer (until the day before the Shavu’ot festival) is found in many Chassidic communities, who follow the teachings of the Kabbalah, especially as it is found in the works of the Kabbalistic traditions of the Ari HaKadosh. For example, this is the practice of Chabad. As I am not fully learned in the Kaballistic tradition, it is probably better that I just cite sources for this practice that are found in the work “Between Pesach and Shavuot” by Rabbi Y Goldstein (which can be found online his website):- “[Not cutting one’s hair until the day before Shavu’ot -] 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; brought in Shaareiy Teshuvah 493:8; Kaf Hachaim 493:13; See also 493:7 The reason: The custom of the Arizal was not to cut his hair throughout the entire period of Sefira until Erev Shavuos. This is not due to mourning but due to that the period of Sefira is a period of severity and judgment. [Poskim ibid] Even according to Nigleh there is reason to not cut hair until Erev Shavuos, as some suspect for all the opinions regarding the mourning period, and since until Erev Shavuos there are opinions who prohibit haircutting [including Lag BaOmer-as rules Michaber ibid] therefore, one who desires to be stringent like all opinions [as rules Admur in 493:7 that doing so is an available option] is to avoid haircuts until Erev Shavuos.” I hope this is of some help. Blessings.