Between the 17th of Tammuz and Tisha B'av
9b. Similarly, during these nine days [between Rosh Chodesh Av and
Tish'ah B'av], it is forbidden to wear or spread [freshly laundered
clothing or linen], even if they were washed before [the nine days]
(1). In honor of Shabbos, however, one may wear [freshly laundered] linen
garments (2) and put out fresh tablecloths and towels, as one does on other
Sabbaths. It is, however, forbidden to spread out clean bed sheets.
A woman that has to wear white undergarments for seven days in preparation
for her ritual immersion, may wash and wear them [during the nine days]
(3). Similarly, a baby's diapers may be washed because they are constantly
being soiled (4).
(1) This prohibition includes all garments which are not worn to absorb
perspiration. Undergarments, socks, and pajamas which are worn directly on
the body may be changed during the nine days, where irritation or
discomfort will develop (Rav Shimon Eider in the name of Rav Moshe
Spreading out freshly laundered bed linen during the nine days is
prohibited; one may not even change tablecloths or towels, unless they have
become soiled ("Summary of the Halachos of the Three Weeks" by Rav Shimon
Eider, pg 10).
Any piece of clothing that has been worn before the nine days for long
enough to cause it to lose its freshly laundered feel, may be worn during
the nine days. For example, if one has worn a shirt for approximately one
hour on a hot day, it almost certainly will have lost its fresh feel, and
thus one would be permitted to wear it during the nine days.
These laws are comparable to those applying to one who is mourning over the
death of a close relative for the first thirty days after the death
("Shloshim") (Kaf HaChaim 170)
(2) Obviously, this is not limited to linen garments, which are only
singled out here because that was what was usually worn on Shabbos.
(3) She may also wash white bed sheets and use them during the nine days.
If she discovers on Tish'ah B'av that she has no clean white underwear, it
is prohibited for her to wash them; rather, she should wear anything which
is clean (stain-free), even if it is colored (Rama 551:3 and Mishna Berura).
(4) The clothing of children who are at the age when they're constantly
making themselves dirty, may be washed even during the week in which
Tish'ah B'av occurs (Rav Shimon Eider: "A Summary of the Halachos of the
Three Weeks" pg 9)