- Peninei Halakha
The rule is that we do not mourn on Shabbat. Therefore, even if the ninth of Av falls out on Shabbat, we postpone the fast to Sunday. Since there is no mourning on Shabbat, on that Shabbat we eat meat, drink wine, serve meals fit even for King Shlomo, and sing Shabbat songs as usual. Obviously, this law also applies when the ninth of Av falls out on Sunday; we treat the Shabbat immediately preceding the fast like any other Shabbat.
However, there is an intermediate time between Shabbat and the start of the fast, when Shabbat has not yet ended but the prohibitions of the fast have already begun to take effect. This happens because of the uncertainty whether one day ends and the next day begins at shki’a or at tzeit. Therefore, the period between shki’a and tzeit – known as bein ha-shmashot – is ambiguous, as it might be considered day and might be considered night. Since there is a mitzva to extend Shabbat, we observe Shabbat until several minutes after tzeit. Consequently, the time between shki’a and shortly after tzeit is considered both Shabbat and part of the fast. During that time, one may not do anything that seems like a mourning custom, because we do not mourn on Shabbat. On the other hand, after shki’a, we avoid activities that are not essential for Shabbat, like eating, drinking, washing, and anointing oneself.
Therefore, we eat se’uda shlishit (the third Shabbat meal) as on any other Shabbat, and we sing songs like we do on any other Shabbat. However, we stop eating and drinking before shki’a, because there is no obligation – from a Shabbat perspective – to continue eating se’uda shlishit after shki’a (sa 552:10, see mb ad loc. 23). It is also proper to refrain from singing joyous songs after shki’a. This is not an expression of mourning because one would not spend all of Shabbat singing joyous songs anyway. We also refrain from washing and applying ointments after shki’a, since we do not bathe on Shabbat anyway. However, one who relieves himself during bein ha-shmashot should wash his hands normally, since otherwise he is, in effect, mourning on Shabbat.
We remain in our Shabbat clothing, keep our shoes on, and continue to sit on chairs and greet one another until a few minutes after tzeit. Then we take leave of Shabbat by reciting the phrase “barukh ha-mavdil bein kodesh le-ĥol (“blessed is the One Who distinguishes sacred from profane”). We then remove our shoes, take off our Shabbat garments, and change into weekday clothes. One should wear clothes that were already worn during the previous week, because one may not wear freshly laundered clothing on Tisha Be-Av.
Many communities have a custom to delay Ma’ariv until about fifteen minutes after Shabbat ends, in order to give everyone time to take leave of Shabbat at home, remove their shoes, change their clothes, and come to the synagogue for Ma’ariv and the reading of Eikha in weekday clothes.
 Some people have a custom to remove their shoes at shki’a, provided that they do so without letting others know that it is for the sake of mourning. Nonetheless, the prevalent custom is to remove one’s shoes only after Shabbat has ended. See Hilkhot Ĥag Be-ĥag, ch. 8 nn. 2, 7, which explains, based on the ruling of the Vilna Gaon, that one may not afflict oneself on Shabbat; therefore, one may wash and apply ointment until Shabbat ends. According to the accepted custom, however, one should refrain from all joyous activities, unless one’s abstention is recognizable as an act of mourning or self-affliction. Therefore, one should not, le-khatĥila, wash or apply ointment during bein ha-shmashot, but one should wash his hands normally after using the bathroom. That is, one should wash his entire hand, not just his fingers as on Tisha Be-Av.