I don’t recall another such occasion in my lifetime. I am not a regular participant in Yom Kippur Katan. But BEH, I will certainly try this afternoon. It is not so unusual for us to hear generalized calls for more davening and mitzvos at a time of tzarah. It is not so usual for different parts of the community to all settle on the same form of avodah she-balev. Today it has happened. The calls to daven Yom Kippur Katan came from R Ahron Leib Shteinman and R Chaim Kanievski in the haredi camp, from R Chaim Druckman, an icon of the Dati-Leumi world, and R David Stav, calling on all Tzohar-related rabbanim and shuls to take part as well.
Remarkably, the Daf Yomi community is now learning Taanis, which deals in no small part with fixed forms of special tefilah response by the entire community to various crises.
May the tefilos of a tzibbur longing for Divine rachamim be answered swiftly.