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.”
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, 5779, 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.