May a person cut or trim their hair during the Three Weeks?
The Sages instituted prohibitions against cutting one’s hair and washing one’s clothes during the week of Tisha Be-Av (Ta’anit 26b). Accordingly, Shulĥan Arukh (551:3) rules that one may not cut one’s hair from the beginning of the week in which Tisha Be-Av falls, and many Sephardim follow this practice. One may not cut even children’s hair if they have reached the age of education, in order to train them to mourn the destruction of the Temple. The custom, moreover, is not to cut the hair of even children who have yet to reach the age of ĥinukh, in order to express the sorrow we feel during this period.
Ashkenazim and some Sephardim – including Jews from Morocco and Djerba, and those who follow Arizal's customs – are stringent and avoid haircuts during the entirety of the Three Weeks ( כף החיים 551:80) Jews from Tunis and Algiers customarily do not cut their hair starting from the first of Av, based on the ruling of R. Yehuda Ayash.
There is disagreement about whether these communities should be stringent about haircuts for children during the entirety of the Three Weeks or only during the week of Tisha Be-Av. In a time of need, one may be lenient until the week of Tisha Be-Av (משנה ברורה 551:82).
This prohibition does not apply only to head and facial hair, but applies to body hair as well. One may, however, trim mustache hair that interferes with one’s eating A woman may also cut her hair if it is beginning to slip out of her head-covering due to its length, and a woman may remove facial or body hair that mars her beauty.