With few exceptions (days when Tachanun isn't said, possibly on the Four Parshiyot), the prayer Av haRachamim, which talks about "the destruction of the Ashkenazi communities around the Rhine River by Christian crusaders during the First Crusade", is recited on every Shabbat. This prayer does not fit with the joyous theme of Shabbat, and as mentioned in Why are some bakashot made on Shabbat? , "prayers of request (bakashot)" are generally prohibited. Why, then, is this prayer recited every week?
(I could understand if it was recited specifically during the dates when the destruction took place, but every week?)