If a person for whatever reason has to eat and drink on Tisha B'Av that comes out on Sunday, and they know this ahead of time, when should they make havdallah on a cos? Should they make Motzei Shabbos like normally (however omit the other parts of havdallah that are omitted when someone would normally make havdallah on Sunday night after Tisha B'Av.) Should they make havdallah (on cos) only right before they will eat or drink. Or Should they wait and make havdallah (on a cos) Sunday night like everyone else?
The Shaarei Teshuva (OC 556) quotes those who require the ill person to recite Havadala "right away after Shabbat", though as Rav Ovadia Yosef (Yechavveh Daat 3:40) explains while citing many sources, this means as soon as they need to eat NOT right away on Saturday night if they can last until later in the day. This is indeed how Rav Yosef rules.
Rabbi Eliezer Barclay Shlita told my father who is going to eat by day to make Havdala just prior to eating in the morning, and omit Besamim and the candle (which was done Motzei Shabbos already).
Zecher Simcha 69 says that one should not make Havdala in such a case unless he needs the wine due to his weakness. However he also brings dissenting opinions that indicate one should make Havdala prior to eating.
Someone told me that Rabbi Leibowitz told him in the name of Rav Shechter that a person who will certainly be eating should make Havdalah on Shabbos after Plag HaMincha.
It was not clear to the Rabbi who told me this if this was a practical Halachic ruling or a theoretical point.