Many have noted the numerous connections between the Seder night and Tisha B'av. For instance, according to the classic "Atbash," the first night of Pesach and Tisha B'av always fall out on the same day of the week. Others suggest that the egg of the Seder night parallels the egg of Tisha B'av. On a deeper level, the two days represent poles of our national experience of exile and redemption.
I thought of this in relation to the unique kinna recited only when Tisha B'av falls out on Motzei Shabbat. We lament that "Vihi Noam nishbat be-motzaei Shabbat," the prayer Vihi Noam is omitted when Tisha B'av falls out on Shabbat. Why is this the case, and why is this omission deserving of its own kinna?
While numerous explanations have been offered for leaving out Vihi Noam (see Tur OC 599 with Beit Yosef), one might suggest that Vihi Noam, which primarily consists of Tehilim 91, also known as the shir shel pegaim, is recited for the purpose of protection as we exit the serenity of Shabbat and enter the uncertainties of the coming week. Thus it is omitted on Leil Tisha B'av, when tragically we cannot pray for protection. As the rabbis teach, tonight we do not anticipate that God will answer our prayers (שתם תפלתי). The prayer's omission is worthy of lamentation because it highlights the dangers inherently associated with Leil Tisha B'av as well as our fears for the futility of our prayers on this night.
Returning to the Pesach-Tisha B'av connection, it turns out that the two are mirror images of one another. On Pesach we don't bother reciting certain parts of Keriat Shema al ha-mita since it is Leil Shimurim (a night of watching); on Tisha B'av we don't bother asking for protection since we are anxious it is not forthcoming. On Pesach we are supremely confident in His presence; on Tisha B'av we experience the anxiety of His absence. (Similarly, on both Pesach - and other holidays - we omit Vihi Noam, just as we do on Leil Tisha B'av, but for very different reasons.)
The linkage between the two moadim, while highlighting the angst of Tisha B'av, also offers a measure of hope. If tonight we struggle with feeling distant from God, we are reminded of the Leil HaSeder, when we brim with confidence that He is with us even in times of darkness. We could not survive as a people if, while mourning, we were not also reminded of the prospect of redemption.