Siman 558

The Tur wrote: It was taught in a Baraita: On the seventh of Av, the idolaters entered the Sanctuary, and they ate and drank and caused destruction on the seventh, eighth, and ninth days. And toward evening, they set fire to it, and it burned until sunset on the tenth day. And this is what Rava said: Had I been there, I would have established it on the tenth, for the majority of the Sanctuary was burned on it (Taanit 29a). And it is found in the Yerushalmi (chapter four, halacha 6): Rabbi Avin fasted on the ninth and the tenth; Rabbi Levi fasted on the ninth and the night of the tenth because he did not have the strength to fast the entire tenth day. And we in this time—our strength is weakened, and even on Yom Kippur, where it would be appropriate to observe two days due to doubt—we are not capable.

Therefore, it is proper and fit not to eat meat on the night of the tenth and the day of the tenth, and likewise not to drink wine. And there are those who are stringent until midday, and no longer, and such is our custom. And if Tisha B'Av falls on Shabbat and is pushed off to Sunday, it is permitted to eat meat and drink wine on Monday, but at night, at the conclusion of the fast, it is forbidden due to the mourning of the day. And for a seudat mitzvah, even if it falls on a weekday, and even on the night following the fast, all those invited are permitted to eat meat and drink wine, for this night is entirely lenient (Magen Avraham). However, a feast before a wedding should not be held then, for it is not a seudat mitzvah (ibid.). And certainly the sound of musical instruments should not be heard there (ibid.). And likewise, one should not wash, haircut, or launder on the tenth day until after midday. And if Tisha B'Av falls on a Thursday, on the following day, Erev Shabbat, everything is permitted for the sake of the honor of Shabbat (ibid.). But to eat meat before midday—it is obvious that it is forbidden, not like those who err in this (see Magen Avraham, subsection 3).