Timeless Torah

Once upon a time, our people enjoyed in tranquility the Holy Land, the City of David, Mount Moriah, the Temple, and the Holy of Holies. Sadly enough, we did not fully appreciate what we had - and so we lost it.
Consequently, four fast days were instituted by the prophets and Sages to commemorate our bereavement. Days when we deprive our body of food and drink, stirring ourselves to repent the sins and shortcomings that twice caused the destruction of G-d's home and our banishment into exile.
The strictest of these fast days is the 9th of Av: Whereas the other three fasts begin at dawn of that day, Tisha B'Av begins at sunset of the previous evening. However, when this fast day coincides with Shabbat, a day when the mitzvah to pleasure ourselves with food and drink supersedes the injunction to fast, the fast is postponed to Sunday. In this context, the Talmud cites Rabbi Yehudah HaNassi's opinion that, “Since it has been postponed, let it be postponed altogether,” and according to the Rabbi, the fast is canceled completely.
Isaiah, the prophet, describes a fast day as, “A day of goodwill before G-d.” In other words, the sadness, the mourning, the recollection of our failings, and the deprivation of the body are not what define the essence of the fast day. At its core, the fast day is a wholly positive phenomenon: a day of opportunity for man to bring himself closer to his Creator.
Why then do we deny ourselves food and drink? Because the material aspect of our lives traditionally sets up barriers that separate us from G-d, we therefore forego some of our body's most basic needs in order to free our soul to take advantage of the, “Day of goodwill before G-d” without hindrance from our physical selves.
This explains why in the era of Moshiach the fast days will be, “transformed into days of gladness and joy.” In that era of perfection, the physical creation will no longer obscure the face of its Creator. On the contrary, it will equal and even surpass the spiritual as an expression of the all-pervasiveness of the divine truth. No longer will there be a need to suppress one's physical desires in order to actualize the positive essence of the fast days. Indeed, they will become festivals, on which the body's pleasure will contribute to the bond between G-d and man.
Shabbat is a weekly taste of the world to come. On fast days that occur on Shabbat, the fast day is stripped of its negative husk and its wholly positive core is exposed. In other words, a fast day occurring on Shabbat is not a clash between two opposing elements in which the stronger element (Shabbat) overpowers the weaker (the fast day), postponing to Sunday the negative aspects of the day. Rather, the essence of Shabbat and the essence of the fast day are fully compatible.
So it's not that the fast day is not being observed on Shabbat - it is - but in a different manner, one that is more consistent with its true function. It is being observed in the fashion in which it will be performed in the perfect world of Moshiach.
This explains Rabbi Yehudah's view that when the 9th of Av falls on Shabbat, the fast day should be “postponed altogether.” For on such a year, Shabbat has enabled us to actualize the quintessential function of the fast day as a, “Day of goodwill before G-d” without the negative externalities it requires on other days. Thus there is no further need, maintains Rabbi Yehudah, for a day of fasting.
In actual practice, we do not follow Rabbi Yehudah's view. For Shabbat is only a taste of a future perfection. We still inhabit a world in which our material involvements tend to obscure, rather than enhance, our spiritual sensitivities. Thus in our present day and age, even on Shabbat, we experience only a semblance of the absolute harmony between man and G-d.
That is why after sampling a messianic Tisha B'Av meal on Shabbat, we must observe another ordinary Tisha B'Av on Sunday. Only then can we fully exploit the days of goodwill as formulated by the Sages.