The Talmud Yerushalmi (Taanit 4:6) declares that when Tishah B'Av occurs on Shabbat, there is no status of shavua she-chal bah (the expanded mourning laws of the week of Tishah B'Av)! This ruling is quoted by numerous rishonim (Rosh (Taanit 4:32) Ramban (Torat ha-Adam, Hilchot Aveilut), Shibbolei haLeket (no. 264, 266), Tur (Orach Chaim 551)), and is even cited in the Shulchan Aruch (Orach Chaim 551:4), although the view (Semag, Semak recorded also in the Tur) that these laws do prevail during the week prior to Shabbat-Tishah B'Av emerges as the normative ruling. The Yerushalmi's prominent view is sometimes cited as a halachic factor (usually in conjunction with others), even by those who accept the Shulchan Aruch's more stringent conclusion. In any case, this perspective commands our attention and demands clarification. While it is eminently reasonable that a Tishah B'Av nidchah (delayed to Sunday, i.e. not the real anniversary) should be treated leniently and differently, it is more difficult to comprehend why the practical inability to observe Tishah B'Av due to Shabbat should preclude the expanded mourning of that week.
The Rosh (and particularly the Tur) implies that this ruling is merely pragmatic: Tishah B'Av is actually observed on Sunday, the beginning of the next week, eliminating any opportunity to implement these laws. However, the question remains why the real anniversary, Shabbat, is not determinative for the prior week aveilut. Moreover, the Shibbolei haLeket formulates the Yerushalmi's conclusion in a manner that underscores a more profound principle ("she-harei ein mitanin bah, shenidchah - because we don't fast on that day, it is delayed", "lamadnu she-ein bo Tishah B'Av klal - we learn that it has in it no Tisha B'Av at all"): a Tishah B'Av anniversary that cannot be demonstratively expressed is fundamentally compromised, also undercutting its extended expressions (shavuah she-chal bah). He does not focus on the nidchah which is not on the anniversary, but rather on the anniversary itself which cannot be fully or properly observed.
This view conveys a profound truth irrespective of whether one normatively adopts the Yerushalmi's actual stance on shavuah she-chal bah. Tishah B'Av is not only the double anniversary acknowledging a traumatic and transformative historical event, the calamitous churban. It is equally an intense expression of collective mourning and an intensive manifestation of identification with the national destiny of the Jewish people. These constitute indispensable elements in internalizing the enormity of the churban and in counteracting it by a process which builds toward redemption. The experiential component that is so central in Tishah B'Av, indeed that distinguishes it even from other tzomot (fast days), is responsible for the expanded aveilut - including shavua she-chal bah - that anticipates the raw and even visceral loss embodied by the concrete aveilut expressions on this terrible anniversary - "moed" (see Eichah, "moed lishbor bachurai").
Previously (The Unique Character of Tisha B'Av), we have suggested that the very concept of "aveilut yeshanah u-de-rabbim" (Yevamot 43b) reflect these motifs. The capacity to engender visceral aveilut collectively and in response to millennia-old national catastrophe, itself reflects fierce commitment and absolute national and historical identification. Perhaps its significance transcends this insight. Chatam Sofer (Torah Moshe, Devarim) notes that the navi speaks in the present tense about the rebuilding of the mikdash and temple: "boneh yerushalayim Hashem...ha-rofei le-shevurei leiv..." He proposes that when the Talmud correlates the capacity to mourn for Jerusalem and the temple and the privilege of rebuilding, it does not merely refer to merit and reward. He concludes that the sincere and heartfelt pain ("shevurei leiv") felt and expressed by the Jewish nation on Tishah B'Av, itself constitutes the building blocks of redemption ("ha-bechiah ve-ha-tzaar shel aveilei ziyon ha-mitablim al churbanah haiynu binyano - the crying and pain of the mourners of Tzion who mourn its destruction constitute its rebuilding"). Elsewhere, Chatam Sofer (and other authorities) posit that Tishah B'Av is a "moed" that also exempts tachanun, not only because of the tradition that this anniversary will ultimately celebrate the rebuilding of the temple, but because the proper halachic-emotional observance of Tishah B'Av eloquently bespeaks of the unity and spirituality of Klal Yisrael, something to celebrate even amidst (and because of) the mourning. Undoubtedly, these very considerations also qualify the anniversary of the churban as the harbinger of the geulah-redemption.
It is noteworthy that the poskim discuss (see, for example, Torat Moshe, Devarim, inyanei Tishah B'Av) the timing of a future celebratory moed marking the tzom of Tishah B'Av (based upon the pesukim in Zechariah) when the anniversary occurs on Shabbat. Some suggest that the celebration will focus not on the churban's anniversary, but on the nidchah day when Klal Yisrael were concretely able to experience and express their grief, albeit imperfectly.
In any case, this perspective may clarify the ruling of the Yerushalmi. The anniversary of the churban is a necessary but insufficient factor to fully accomplish the full measure of Tishah B'Av's agenda. Klal Yisrael's capacity to grieve - internally and demonstrably - constitutes a core component not only in marking the tragedy, but in redressing it and building to a geulah sheleimah. May we merit to experience it speedily in our days.