סימן קכה סעיף א
תִּשְׁעָה בְּאָב שֶׁחַל בְּאֶחָד בַּשַׁבָּת אוֹ שֶׁחָל בַּשַׁבָּת וְנִדְחָה לְאֶחָד בַּשַׁבָּת, אוֹכְלִין בַּשַׁבָּת בָּשָׂר וְשׁוֹתִין יָיִן. וַאֲפִלּוּ בִּסְעוּדָה שְׁלִישִׁית שֶׁלְּאַחַר מִנְחָה, מֻתָּר בַּכֹּל, אַךְ לֹא יֵשֵׁב אָז בִּסְעוּדַת חֲבֵרִים. אִם חָל בְּרִית מִילָה, יַעֲשׂוּ הַסְּעוּדָה קֹדֶם מִנְחָה. אֲבָל מֻתָּר לֶאֱכוֹל עִם בְּנֵי בֵיתוֹ, וְיָכוֹל לְבָרֵךְ בְּזִמּוּן. וְצָרִיךְ לְהַפְסִיק מִבְּעוֹד יוֹם, כִּי בְּבֵין הַשְּׁמָשׁוֹת אָסוּר בַּאֲכִילָה וּשְׁתִיָּה וּרְחִיצָה. אַךְ הַמִּנְעָלִים לֹא יַחֲלוֹץ עַד לְאַחַר בָּרְכוּ. וּשְּׁלִיחַ הַצִבּוּר חוֹלֵץ קֹדֶם שֶׁמַּתְחִיל וְהוּא רַחוּם, כְּדֵי שֶׁלֹּא לְבַלְבֵּל דַּעְתּוֹ, וְיֹאמַר מִתְּחִלָּה בִּרְכַּת הַמַּבְדִּיל בֵּין קֹדֶשׁ לְחֹל בְּלֹא שֵׁם וּמַלְכוּת (תקנב תקנג).
If Tishah beAv occurs on Sunday, or if it occurs on Shabbos and is deferred to Sunday, you may eat meat and drink wine on Shabbos.1It is forbidden to refrain from eating meat on Shabbos, because of mourning. (Mishnah Berurah 552:23) Even at the third meal, after Minchah, you may eat everything2It is said in Shulchan Aruch, that you are permitted to serve at this meal, even like the royal feast of King Solomon. but you should not have this meal in the company of friends.3Bechor Shor disagrees. He rules that if you are accustomed to eat in the company of friends every Shabbos, you may do so on this Shabbos as well, because, otherwise, it would be considered mourning in public, which is forbidden on Shabbos. (Ibid.) If a bris is to be held, the meal should be served before Minchah. But you are permitted to eat the third meal with your family, and you may say Birkas Hamazon with zimun. You must finish the meal, while it is still daytime,4I.e. before sunset. (Ibid. 552:24) because at twilight you are forbidden to eat, drink or wash. However, you should not take off your shoes until after Barechu is said. The Chazzan takes off his shoes before saying Vehu Rachum [He, the Merciful One], so that he should not get confused; but before [taking off his shoes] he should say: Baruch hamavdil bein kodesh lechol [Blessed is He Who separates between the holy and the secular], without mentioning Hashem's Name and Kingship [Hashem Elokeinu melech haolam].
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)