Begins at sundown on May 4.
The full name of the day commemorating the victims of the Holocaust is “Yom Hashoah Ve-Hagevurah”— in Hebrew literally translated as the "Day of (remembrance of) the Holocaust and the Heroism." It is marked on the 27th day in the month of Nisan — a week after the end of the Passover holiday and a week before Yom Hazikaron (Memorial Day for Israel's fallen soldiers). It marks the anniversary of the Warsaw Ghetto uprising.
The date was selected in a resolution passed by Israel's Parliament, the Knesset, on April 12, 1951. Although the date was established by the Israeli government, it has become a day commemorated by Jewish communities and individuals worldwide. The day's official name - Holocaust and Heorism Remembrance Day- was made formal in a law enacted by the Knesset on August 19, 1953; on March 4, 1959, the Knesset passed another law which determined that tribute to victims of the Holocaust and ghetto uprisings be paid in public observances.
Source Jewish Virtual Library