minhag - Holocaust Rembrance Day(s)?

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.)}

Answer

January 27th is probably the most universally accepted non-Jewish holocaust remembrance day. It is the anniversary of the day that Soviet Troop liberated Auschwitz-Birkenau in 1945. It was designated by the UN in 2005 and as of 2004 at least 12 countries have some type of official observance on this day. Israel has designated this day, not as a holocaust remembrance day, but as a day to mark the struggle against anti-semitism. Ceremonies are held at the United States Holocaust Museum in Washington DC and in Israel's Yad VaShem. This date has a specifically non-religious character.

Nissan 27 is the most universally accepted Jewish holocaust remembrance day. The date is tied to the Warsaw Ghetto uprising, but is not the actual anniversary. The actual anniversary takes place on the 14th of Nissan, but due to its proximity to Passover the date was moved to the 27th of Nissan. It was officially established by Israel's Prime Minister and President in 1953. Most communities that have a dedicated Holocaust memorial service (generally "Modern" Orthodox) will do so on Yom HaShoah (27 Nissan), while those who remember the Holocaust together with other national tragedies (generally "Yeshivish" Orthodox) will do so on Tisha B'Av or Asara B'Tevet. Some communities will also hold educational events (but not specifically religious services) on Yom HaShoah.

Many Jewish communities will hold educational and other non-religious events on the days designated by their host-countries as "Holocaust Memorial Day" or other days which may be locally designated as such or on the International Holocaust Remembrance Day (Janaury 27th).