From the United States Holocaust Memorial Museum:
The internationally recognized date comes from the Hebrew calendar and corresponds to the 27th day of Nisan on that calendar. It marks the anniversary of the Warsaw ghetto uprising. In Hebrew, Holocaust Remembrance Day is called Yom Hashoah. When the actual date of Yom Hashoah falls on a Friday, the state of Israel observes Yom Hashoah on the preceding Thursday. When it falls on a Sunday, Yom Hashoah is observed on the following Monday.
In Israel, on Holocaust Remembrance Day, a mournful siren’s wail calls all for a moment of silence. This video, of one of Israel’s busiest freeways, demonstrates the reverence of those who will not let the knowledge of the world’s worst atrocities fade, lest they one day are repeated. Click on the image above, or jump through the break to see.
Imagine if America did something similar on each September 11th? Yeah, right.