The kina (elegy) "Mi yiten roshi mayim" recited on 9 Av was composed in commemoration of the events of the First Crusade as it passed through the Rhineland. It includes the following lines (freely translated):
And because one ought not add a day to the calendar commemorating destruction...
I will instead arouse my cries of "woe!" today [i.e., 9 Av]
One could, one might think, dismiss this viewpoint as one that was not accepted as normative halacha/hashkafa. One could, except that there are those who applied this consideration, in practice, to the Holocaust, saying one should not add a day to the calendar (e.g., Yom Hashoa) commemorating it. (I see this argument attributed to Rabbi Joseph B. Soloveitchik.)
My question is, what do such opinion-holders do with 20 Sivan, which has a history of great rabbis' support, and commemorates post-churban tragedies?