Siman 558
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א בְּתִשְׁעָה בְּאָב לְעֵת עֶרֶב הִצִּיתוּ אֵשׁ בַּהֵיכָל וְנִשְׂרַף עַד שְׁקִיעַת הַחַמָּה בְּיוֹם עֲשִׂירִי, וּמִפְּנֵי כָּךְ מִנְהָג כָּשֵׁר שֶׁלֹּא לֶאֱכֹל בָּשָׂר וְשֶׁלֹּא לִשְׁתּוֹת יַיִן בְּלֵיל עֲשִׂירִי וְיוֹם עֲשִׂירִי. הַגָּה: וְיֵשׁ מַחְמִירִין עַד חֲצוֹת הַיּוֹם וְלֹא יוֹתֵר (הַגָּהוֹת מַיְמוֹנִי); אִם חָל תִּשְׁעָה בְּאָב בְּשַׁבָּת וְנִדְחָה לְיוֹם א', מֻתָּר לֶאֱכֹל בָּשָׂר וְיַיִן יוֹם ב', אֲבָל בַּלַּיְלָה אָסוּר מִפְּנֵי אֲבֵלוּת שֶׁל יוֹם (מַהֲרִי"ל).
1 On Tisha Be-Av around afternoon time the sanctuary caught fire, and burned until the sun set on the tenth, [subsequently] for this reason the proper custom is not to eat meat, nor to drink wine on the night of the tenth or the day of the tenth. Rema: There are those who are strict until midday and no longer (Hagehot Maymoni). If Tisha be-Av fell on Shabbat and was pushed off until the first day [of the week], it is permitted to eat meat and drink wine on the second day [of the week], but the night is forbidden because of the mourning of the preceding day (Maharil).