The Shulchan Aruch (Orach Chaim 551:16) records a custom practiced by Ashkenazim of not bathing or showering during the Nine Days, from Rosh Chodesh of the month of Av until the end of the Tisha B’Av mourning. Rav Moshe Soloveichik explained the custom as follows:
When someone’s relative dies, there are three periods of mourning he may have to observe: “Shivah,” the week of intensive mourning right after burial; “Shloshim,” a thirty-day period (including the week of Shivah) in which one observes various mourning restrictions; and when mourning for a mother or father, “Yud Bet Chodesh,” a year (including the week and month of Shivah and Shloshim) of limited mourning restrictions.
Tisha B’Av (this year, July 20th) is a day of such intense mourning that we observe all of the restrictions of Shivah, in addition to fasting. The Ashkenazic custom is to adopt many restrictions of Shloshim during the Nine Days – from Rosh Chodesh Av until after the Tisha B’Av mourning – and restrictions of Yud Bet Chodesh during the earlier part of the Three Weeks (beginning this year on June 29th).
Rav Moshe Soloveichik explained that the Shulchan Aruch (Yoreh Deah 381:1) records the original Ashkenazic custom not to shower or bathe for the entire Shloshim period. Therefore, the custom was to similarly refrain from showering or bathing during the Nine Days. However, since nowadays the custom is to shower immediately after Shivah and not to be stringent in this regard during the Shloshim period, therefore there is no reason to refrain from bathing during the Nine Days. The old Nine Days custom no longer applies because it was based on a mourning custom that is no longer observed.
Adapted from Shiurei Harav on Mourning and Tisha B’Av, based on the lectures of Rabbi Joseph B. Soloveitchik. This and other books are available for purchase at oupress.org.