Are there any restrictions on Shabbat when the Fast of Tish'a B'Av falls out on Shabbat?
We do not mourn on Shabbat. Therefore, even if Tish’a B’Av falls out on Shabbat, we postpone the fast until after Shabbat. On that Shabbat, we may eat meat, drink wine, and serve a meal fit for a king. We also sing Shabbat songs as usual. This law would also applies when Tish’a B’Av falls out on Sunday; we treat Shabbat preceding the fast like any other Shabbat.
However, there is an intermediate time between Shabbat and the fast, during which Shabbat has not yet ended but the prohibitions of the fast have already begun. This happens because we are unsure when one day ends and the next day begins – at sunset or when the stars emerge. Therefore, the period between sunset and the emergence of the stars is ambiguous, being possibly day and possibly night. It is called “bein hashmashot” (twilight). Since there is a mitzvah to add [time] onto Shabbat, the holy day continues until a few minutes after the stars emerge. Consequently, the time between sunset and shortly after the emergence of the stars is both Shabbat and Tish’a B’Av. During that time, it is forbidden to do anything that would appear like a custom of mourning because we do not mourn on the Sabbath. On the other hand, after sunset we avoid doing anything that is not necessary for the sake of Shabbat like eating, drinking, washing, and anointing.
Therefore, we eat the third seudah shlishit like we do on any other Shabbat. However, we stop eating and drinking before sunset because there is no obligation – from a Shabbat perspective – to continue eating seudah shlishit after sunset, We also refrain from washing and anointing ourselves after sunset; after all, we do not bathe or anoint ourselves on Shabbat anyway. However, one who relieves himself during bein hashmashot should wash his hands normally.
We remain in our Sabbath clothing, keep our shoes on, and continue to sit on chairs and greet each other until a few minutes after three, midsized stars appear in the sky. Then, we end Shabbat by say Baruch ha’mavdil bein kodesh le’chol. Afterwards, we remove our shoes, take off our Sabbath garments, and change into weekday clothes. One should wear clothing that was already worn the previous week because one may not wear freshly laundered clothing on Tish’a B’Av