The Holocaust was a state-sanctioned vicious murder of millions of people across Europe by Nazi Germany. It occurred from 1941 to 1945. About 6 million Jews across Europe, as well as Jehovah's Witnesses, Roma, homosexuals, people with disabilities, Poles, Soviet prisoners of war, Soviet citizens, and others who did not align themselves with the Nazis' beliefs. Victims were subjected to torture and other acts of violence.
Yom HaShoah, or as it is called in English, Holocaust Remembrance Day is an annual commemoration of the six million Jews who died, and the heroes and survivors of the Holocaust. This year, it takes place sundown on April 27 to sunset on April 28. The date "changes" year by year because like other Jewish holidays, it is observed using the Hebrew calendar. Its Hebrew date is the 27th of the month of Nissan. However, if it will coincide with the Jewish Sabbath, then Yom HaShoah is then to be observed the following day.
In the United States, Days of Remembrance runs from the Sunday before Yom Hashoah through the following Sunday.
Listed below are some articles that describe how Holocaust Remembrance Day is observed.