On Shabbos, we should also make celebrations for finishing tractates of Talmud. These celebrations should include Tzedakah; on Shabbos, when we may not give money, we can give Tzedakah by inviting people to share our food and drink.
This year Tishah B'Av is after Shabbos. If, G-d forbid, Moshiach is delayed, then starting from the afternoon before Tishah B'Av, we are only allowed to study certain subjects of Torah that are connected to mourning. We can complete the tractate that deals with mourning (euphemistically called "Smachos") or the tractate "Moed Katan" if we have already reached the last section, which is about mourning. Since it is Shabbos, though, we can and should have a lavish meal with meat and wine like King Solomon's royal feasts, but must stop eating before sunset. It's a taste of the transformation of Tishah B'Av into a joyous holiday in the Redemption through Moshiach.
On Tishah B'Av, we can also complete the above-mentioned tractates, but without a meal.
The main thing is that immediately we will see the Rebbe King Moshiach Shlita bringing the true and complete Redemption, and this year Tishah B'Av will be a great holiday! We will still continue the custom of completing Talmud tractates and celebrating in the Redemption too!
(See talk of the Lubavitcher Rebbe King Moshiach Shlita, Devarim 5751 (1991) and more)
