Between the 17th of Tammuz and Tisha B'av
11. Certain women follow the custom of not arranging the threads on the loom for weaving [during the nine days between Rosh Chodesh Av and Tish'ah B'av]; this is because the threads meant for the warp are referred to as "sh'si" in Hebrew (from the root meaning "foundation"). Once the "Even haSh'siyah" ("foundation stone") in the Beit Hamikdash (Temple) was destroyed, they accepted this stringency upon themselves (as a remembrance). Women who follow this custom, should not be permitted [to engage in this activity].
Halacha-Yomi, Copyright (c) 2002 Project Genesis, Inc.