Q. In a shul were every Shabbos eve after Kabbolas Shabbos and Maariv, there is a rikud, and some say that on this Shabbos which coincides with Tisha Beav it should not be done, would that not be aveluth befarchesia if we omit it, as we always do it?
A. The prohibited avelus befarhasia is not only the performance of a public act of mourning on Shabbos, such as reciting Kinos on Shabbos (Yalkut Yosef – Moadim p.566) but also the refraining from doing something that we would usually do overtly,. such as not changing the weekday poroiches or ones everyday clothes for the singular ones used on a regular Shabbos (Piskey Teshuvos 551:4) or refraining from calling a mourner for an aliyah on Shabbos, when he is called on every single Shabbos.
However, in this particular case of dancing a rikud on Shabbos, as most Shuln do not follow this tradition, and it is sometimes omitted in that Shul too, it would not be a case of avelus befarhasia, if not done on this Shabbos Chazon that coincides with Tisha Beav.