When the 9th of Ab falls on Shabbat as it does this year, the observance of Tisha B’Ab is delayed until after Shabbat. As such, Habdala is recited on Sunday night, at the conclusion of Tisha B’Ab, and not on Mosa’eh Shabbat.
In this case, one who is required by doctor’s orders to eat on Tisha B’Ab must recite Habdala before eating. Since eating is forbidden after Shabbat before one recites Habdala, those who need to eat on Tisha B’Ab which is observed on Sunday must first recite Habdala. According to many Halachic authorities, a person in this situation recites Habdala just before he needs to break the fast. If, for example, a person is able to fast until 9am on Sunday morning, then, according to this view, he recites Habdala at that time, when he needs to break his fast. According to this opinion, since the person does not need to eat on Mosa’eh Shabbat, there is no reason to recite Habdala then, and Habdala should be recited only when he needs to break his fast.
Hacham Ovadia Yosef, however, disagreed. He felt that if a person knows for certain when Tisha B’Ab begins that he will not be completing the fast, as his medical condition does not allow him to fast for the entirety of Tisha B’Ab, then he should recite Habdala at its usual time, on Mosa’eh Shabbat. If it is definite that one will need to recite Habdala and eat at some point over the course of Tisha B’Ab, then he should recite Habdala on Mosa’eh Shabbat, as usual. However, if the individual is uncertain whether he will need to break the fast, as this depends on how he feels over the course of Tisha B’Ab, then he should not recite Habdala until he eats. Since there is a chance that he will complete the fast, he should not recite Habdala on Mosa’eh Shabbat, and should instead wait until he realizes he cannot complete the fast, and then recite Habdala when he needs to eat.
If a person is able to complete the fast, but needs to drink water at some point during the day (as in the case of patients with kidney disorders), then he does not recite Habdala before drinking. Even on an ordinary Mosa’eh Shabbat, the Shulhan Aruch rules that it is permissible to drink water before Habdala. Accordingly, when Tisha B’Ab is observed on Sunday, those who need to drink on Tisha B’Ab do not need to first recite Habdala. One recites Habdala on Tisha B’Ab in this case only if he needs to eat.
When one recites Habdala on Tisha B’Ab, he omits the festive verses which are normally recited as an introduction to Habdala, and also omits the Beracha over the Besamim. He begins with "Kosh Yeshuot Esa," and recites the Beracha over the wine and the Beracha of "Ha’mabdil."
Summary: When Tisha B’Ab begins on Mosa’eh Shabbat, Habdala is recited on Sunday night, at the conclusion of Tisha B’Ab. However, one who knows for certain when Tisha B’Ab begins that he will not be able to complete the fast, due to a medical condition, recites Habdala on Mosa’eh Shabbat, even if he will not need to eat until sometime the next day. If one is unsure whether or not he will be able to complete the fast, then he does not recite Habdala on Mosa’eh Shabbat, and if he needs to eat during Tisha B’Ab, he recites Habdala before eating. One who is able to complete the fast but needs to drink water does not recite Habdala before drinking. When one recites Habdala on Tisha B’Ab, he omits the festive verses which are normally recited as an introduction to Habdala, and also omits the Beracha over the Besamim.
