Donating hair on the three weeks: She’ela, Teshuva (https://www.matan.org.il/en/qna/donating-hair-on-the-three-weeks/)

She'ela (Question)

Can I donate my hair to Zichron Menachem (to make wigs for children with cancer) during the three weeks? I want to do it in honor of my mother’s Yartzeit which is 24th of Tammuz.

Teshuva (Responsum) from Rabbanit Surale Rosen

The earliest reference we have is ספר המנהגים לרבינו אייזיק טירנא (app. 1400) where he brings the minhag not to have a hair cut from 17th of Tammuz till after Tisha B’Av. The Remah in Orach Chayim 551:4 mentions this Ashkenazi custom together with the minhag not to wash clothes from Rosh Chodesh Av as opposed to the Shulkhan Arukh who rules that it’s only on the actual week of the fast…

The prohibition to have a haircut during the entirety of the 3 weeks is not explained but the Remah brings it as הלכה למעשה that we know has been kept for generations. Therefore, I would advise you to cut your hair before 17th of Tammuz and then make the actual donation on the date of your mother’s Yahrzeit.