Q. On this year that Tisha BeAv is on motzei Shabbat, can one continue to wear for Maariv and Eicha the Shabbat clothes? What if one usually keeps Rabbenu Tam’s time?

A. Remoh (O.H. 553: 2) rules that when T’Beav falls on Motzei Shabbos, one should remove his leather shoes after answering borchu at the beginning of ma’ariv, and not before the shkiah, as we normally do. Mishna Berura (ibid. 6) explains that it is prohibited to show openly any demonstration of avelus (mourning) on Shabbos.
Eishel Avrohom (Tanina 551) maintains that in principle there is no prohibition to continue wearing Shabbos clothing already dressed at the start of T’BeAv as opposed to leather shoes. One is only enjoined on not putting these garments on during T’BeAv. Nitei Gavriel (Ben Hametzarim 94: n. 15) relates that the Lubavitcher Rebbe would continue wearing his Shabbos clothing. Equally, other of the prominent sages of Yerushalaim wore their shtraimels until after Eicha and the Kinos recited on Motzei Shabbos.
However, a common tradition today is to delay davening ma’ariv; so permitting people to change shoes and clothing at home after saying Boruch Hamavdil ben Kodesh Lechol at the end Shabbos and returning to shul. Others prepare the clothing and shoes, and bring them to shul before Shabbos.

Horav Shlomo Miller’s Shlit”a opinion is that one may keep his Shabbos clothing on until the Rabbeinu Tam’s time, even if he only does so lechumra. Horav Dovid Pam Shlit”a quoted on the Rosh Hakolel’s name that you may also change to everyday clothing (excluding shoes) forty five minutes after sunset. However, one should first make sure that there are no muktze items inside the pockets
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Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a