April 19th is Yom HaShoah. As with most Jewish holidays, it begins at sundown the evening before.
Yom HaShoah is a new holiday, designated by the Knesset (Israeli Parliament) in 1953, commemorating the victims of the Holocaust. The full name is Yom HaShoah veHagevurah, the Day of Remembrance of the Holocaust and the Heroism. It occurs each year on the 27th of Nisan, except when adjacent to Shabbat, when it moves one day.
In Israel, a ceremony is held at Yad Vashem at sundown. Flags are flown at half mast. The entire day is very solemn. In the morning, sirens are sounded throughout Israel for two minutes when the entire country stops to pay silent tribute. Even on highways, drivers stop their cars and wait in silence. Public entertainment venues are closed by law.
Many Orthodox do not commemorate this date, instead using the 10th of Tevet or Tisha b’Av for remembrance.
Outside of Israel, commemorations range from synagogue services to community vigils and educational programs. Rituals are still being created and vary widely but usually include the lighting of a Yahrzeit (memorial) candle and recitation of the Mourner’s Kaddish.
In 1980, the US Congress designated an eight day period of The Days of Remembrance of the Victims of the Holocaust (DRVH), beginning the Sunday before Yom HaShoah and continuing until the following Sunday. The US Holocaust Memorial Museum designates a theme for each year’s programs. For 2012, the theme is Choosing to Act: Stories of Rescue, and DRVH is from April 15-22.
In 2005, the United Nations also designated an International Holocaust Remembrance Day for the 27th of January.
Most of this information and more can be found at some of the following web sites: Wikipedia, Jewish Virtual Library, and the US Holocaust Memorial Museum.