At sunset today (Sunday, April 7), Jews worldwide will begin observing Yom HaShoah, or Holocaust Remembrance Day. A relatively recent modern addition to the Jewish calendar (having been established in 1953), this is a solemn memorial day commemorating the approximately six million Jews who fell victim to the Nazi Holocaust during World War II.

Why does Yom HaShoah begin at sunset? Judaism traditionally reckons a “day” as beginning not at 12:00 midnight, nor at dawn, but instead at sunset. Yom HaShoah will therefore technically begin at sunset this evening. It will conclude about 24 hours later, at sunset on the evening of Monday, April 8.

Yom HaShoah always begins on the 27th day of the Hebrew month of Nisan (unless that date falls immediately adjacent to the Sabbath; if so, its observance is bumped by a day). However, that fixed starting date of 27 Nisan on the Jewish religious calendar does not always coincide with April 7 on the secular Western (Gregorian) calendar.

The traditional Jewish calendar is a lunar calendar, which means that it counts and calculates its lunar months somewhat differently from how the widely-used Gregorian calendar (which is a solar calendar) reckons its own months. This means that there is a certain amount of built-in “drift,” from year to year, between the two calendars.

Last year, for instance, Yom HaShoah (always 27 Nisan on the Hebrew calendar) began at sunset on April 18, 2012. Next year, by contrast, Yom HaShoah will begin at sunset on April 27, 2014.

The Hebrew word yom means “day” (as in Yom Kippur, “Day of Atonement”), and shoah is Hebrew for “calamity,” “catastrophe,” “disaster,” or “destruction.” In modern usage, that term has become almost synonymous with the Holocaust, the mass extermination of some six million Jewish men, women, and children at the hands of the Nazis.

Yom HaShoah (“Day of the Holocaust”) is thus a time of remembrance for that tragic and horrific program of genocide, which systematically slaughtered approximately two-thirds of the European Jewish population (amounting to about one-third of the total world Jewish population).

There is no standard or traditional form of greeting (that I know of) for Yom HaShoah. However, perhaps one well-known and oft-used slogan might be particularly apropos: “Never Again.”