The following rules apply to any year on which Tisha B’Av is observed on Sunday—whether it originally fell on Sunday, or whether it fell on Shabbat and the fast was postponed until Saturday night.
On Shabbat, all public displays of mourning are strictly prohibited. On this day we eat, drink and rejoice as is customary—and even more so.
There are two exceptions: On this day we eat, drink and rejoice as is customary—and even more so(a) If Shabbat is actually the 9th of Av, then marital relations are forbidden in Ashkenazi tradition.1 (b) In all cases when Tisha B’Av is observed on Sunday, it is forbidden to study Torah starting with Shabbat midday (aside for those sections of Torah which are permitted to be studied on Tisha B’Av). As such, on this Shabbat we do not recite a chapter of Ethics of the Fathers, as is the custom in many communities on summertime Shabbat afternoons.
No mournful “separation meal” is conducted before the fast. Instead, shortly before sunset we partake of a sumptuous and joyous pre-fast meal. Care must be taken, however, that this meal ends before sunset.
We sit on chairs of regular height and wear normal footwear until nightfall. Only washing, eating and drinking are prohibited starting with sunset.
Havdalah is recited on Sunday night.2 In the evening prayers, the usual Shabbat night insertion, “Atah Chonantanu,” is included. The prayer “Vihi Noam” is omitted. Those who have not recited the evening prayers should say, before doing any activity that is forbidden on Shabbat, “Baruch hamavdil bein kodesh l’chol” (“Blessed is He who separates between the holy [day of Shabbat] and the mundane [weekday]”).
Sometime on Saturday night (ideally right before the reading of Eicha), kindle the havdalah candle and recite the appropriate blessing. (We do not recite the blessing of the spices.)
Immediately after the “Barchu” passage is recited in the Saturday night prayer service, remove your leather shoes and don non-leather footwear.
Recite the havdalah on Sunday night before eatingWe sit on chairs of regular height and wear normal footwear until nightfall—omitting the blessings on the spices and candle. When 9 Av is on Sunday, if possible, the havdalah wine or grape juice should be given to a child—younger than bar/bat mitzvah age—to drink.
If the ninth of Av falls on Shabbat, in which case the fast is delayed until the tenth, many of the restrictions applicable to the Nine Days end when the fast ends, and havdalah wine, music, bathing and haircutting are permitted. We do not eat meat or drink other wine until the next morning, however.
This is because abstaining from relations does not constitute a public display of mourning. However, on this Shabbat only actual marital relations are banned (as opposed to Tisha B’Av itself, when all forms of intimacy are forbidden). This prohibition does not apply if Friday night is mikvah night.
If there’s an ill person who needs to eat during the fast, he or she should recite the havdalah before eating.
Source: Chabad.org