1) It is forbidden to show signs of mourning on Shabbat. Yet, some of the laws of Tisha B’av – which include symbolic acts of mourning – become applicable on the afternoon before the fast begins, which this year is on Shabbat. What to do?
2) Until shkiya ha’chama, there should be no difference in one’s Shabbat observance. Seudat shlishit with as many cooked foods as one desires and joyous singing is entirely appropriate (though there are some who say one should sit separately during seudat shlishit to defer ever so slightly to Tisha B’av; this, however, is not necessary according to most poskim). At 7:26 PM, one should stop all eating, as explained below:
3) However, between shkiya hachama and tzeit hakochavim, it becomes more complicated. This is a period of doubt; is it still Shabbat or is part of the next day, the fast day? The halacha is that one should continue to treat the day fully as Shabbat but should not engage in any activities not required by Shabbat that would interfere with Tisha B’av. For example, one should not eat, wash themselves or sing zmirot during this time (i.e. 7:26 PM), for although we still treat the time as if it were Shabbat, there is no obligation to eat, sing or wash all of Shabbat. On the other hand, changing clothes and removing one’s leather shoes would not be appropriate, for in doing so, one is suggesting that they have begun the mourning process.
3) Rather, one should wait until Shabbat officially ends and then say “Hamavdil ben kodesh l’chol” and remove one’s shoes and change from Shabbat clothing into weekday clothing. (Ma’ariv at shul will be called for a later than usual time to allow this to happen).
4) Regarding havdalla, one should not say it – the wine and hamavdil b’racha – until after the Tzom. After all, one cannot say the b’racha on wine until that time anyway. However, some people say the b’racha over the candle on Motzei Shabbat because that is connected specifically to the first Motzei Shabbat during creation. One could make this b’racha at shul when they bring a candle for the reading of Aicha. There is no b’racha for b’samim this havdalla.
5) If one is sick and unable to fast, such a person should say havdalla after Shabbat ends and not wait until after the tzom. However, they should not say it over wine, but rather some other important liquid.