What is with the different Holocaust Remembrance Days?
When I was a kid, my entire community observed Yom HaShoah. This came about approximately one week after Pesaḥ. Although I'm not totally sure, I think they must have followed the date observed in Israel, which is 27 Nisan. The particular date was of extreme importance to the organizers, however, because it was essential that all the various Jewish organizations joined together as one community to observe this solemn day, and the March of the Living was also scheduled around that date.
In college, or more precisely, in Yeshivah, I learned that there was some early opposition from religious groups to establish a date for Yom HaShoah, since the fast of the Tenth of Teveth was considered the catch-all for Jewish national tragedies, as is the Sefirah period*.
Nowadays, though, it seems that there are several other, non-religious, internationally recognized dates for Holocaust Remembrance.
So what's going on exactly? What's the history of these divergent observances, and who actually observes them? It still seems to me that Yom HaShoah is the dominant date observed by most people, but is that really true? Recently there was some controversy over an offensive (allegedly antisemitic) cartoon published on one of these dates. It made waves because of the double impact of resembling a blood-libel cartoon from the Nazi era and being published on "Holocaust Remembrance Day" (I think the UN-sanctioned date). But other than its own distastefulness and offensiveness, was this really an issue for most people? I mean, did anyone who is not in tune with internationally-recognized holidays make this connection because they personally observe that date?
*(As explained to me, the Three Weeks, etc., are regarded as unique dates of suffering, not to be associated with general tragedies that didn't befall the Jewish people on those specific dates.)