The Baraita in Masechet Ta’anit (30a) states that our Sages prohibited five things on Tisha Be’av: Eating and drinking, washing one’s self, rubbing one’s self with oils or lotions, wearing leather shoes, and marital relations.
Our Sages said (Ta’anit 30b): “One who mourns for Jerusalem shall merit seeing its joy, but one who does not mourn over Jerusalem shall not merit seeing its joy.”
Commencement of the Five Categories of Abstinence
On Erev Tisha Be’av, the five categories of abstinence take effect immediately at sunset. For this reason, one must remove one’s leather shoes immediately at sunset (it is nevertheless permissible to wear non-leather shoes). However, on years like this, 5781, when Tisha Be’av falls out on Motza’ei Shabbat, one should not remove one’s leather shoes immediately at sunset in honor of Shabbat; rather, one should leave one’s leather shoes on until after nightfall which is approximately twenty minutes after sunset. One should then recite, “Baruch Ha’Mavdil Ben Kodesh Le’Chol,” remove one’s leather shoes, change into weekday clothing, and head to the synagogue in order to pray Arvit and recite Eicha and other lamentations.
As a result of all of the above, Maran Rabbeinu Ovadia Yosef zt”l writes that it is proper to announce in the synagogue and notify the public that Arvit services will commence a half-hour after sunset and not beforehand in order to allow the congregants to change from Shabbat clothing into weekday clothing, to remove their leather shoes, and to make their way to the synagogue wearing weekday clothes which are permitted to be worn on Tisha Be’av.
Clothing for Tisha Be’av
Just as on all other years, it is forbidden to wear laundered clothing during the week preceding Tisha Be’av, it goes without saying that on Tisha Be’av itself, one may not wear freshly-laundered clothing. Thus, if one finds it difficult to wear the same shirt one wore on Shabbat on Tisha Be’av, one must make sure to prepare clothing for Tisha Be’av by wearing them for about a half-hour and then putting it aside for Tisha Be’av. One should wear these clothes immediately after Shabbat has concluded, before Arvit prayers of Tisha Be’av.