When and how do we make havdalah when Shabbat is the eve of the fast of Tishah b'Av as it is this year? After all, we cannot drink the wine as Tishah b'Av has already started. There are three main opinions. (I am indebted for this analysis to Rabbi Yosef Zvi Rimon.) One opinion is that of the Geonim (cited by Tosafot); havdalah is said on Sunday night at the end of Tishah b'Av. The second opinion is that of the Manhig (Rabbi Abraham ben Nathan): havdalah is recited on Motzei Shabbat, and the wine is given to a young child to drink. The third opinion is that of the Ramban that there is no havdalah. Since havdalah was set aside on Motzei Shabbat, it is set aside forever.
The authoritative Shulchan Aruch decides that we make havdalah on Sunday night, after Tishah b'Av (556:1). The blessing over light is, however, said on Motzei Shabbat. The blessing over spices is not recited at all, for it is inappropriate for a time of mourning.