At sunset today (Monday, March 25), Jews worldwide will begin observing the eight-day festival known in Hebrew as Pesach, and in English as Passover.

Why at sunset? Because Judaism traditionally reckons a “day” as beginning not at 12:00 midnight, nor even at dawn, but instead at sunset. Pesach or Passover will therefore technically begin tonight at sunset. It will then last for eight days, before concluding at sunset on the evening of Tuesday, April 2. No work is permitted during the first two and the last two days of this eight-day period.

(In Israel, however, Pesach or Passover instead lasts for only seven days; no work is permitted on just the first and the last day of that period.)

Passover always begins on the fifteenth day of the Hebrew month of Nissan. However, that fixed starting date of 15 Nissan on the Jewish religious calendar does not always coincide with March 25 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, Passover (always 15 Nissan on the Hebrew calendar) began at sunset on April 6 and ended at sunset on April 14, 2012. Next year, by contrast, Passover will begin at sunset on April 14 and end at sunset on April 22, 2014.

Regardless of when 15 Nissan falls in relation to the Gregorian calendar, Passover commemorates the ancient Israelites’ years of slavery in Egypt, and celebrates their subsequent liberation from bondage and departure from Egypt under the leadership of Moses (as described in the biblical book of Exodus, and as popularly depicted in Cecil B. DeMille’s Hollywood epic The Ten Commandments, often broadcast around this time of year).

The name “Passover” derives from the account in Exodus describing how, when God dispatched the angel of death to slay the firstborn children of Egyptian families (the last of ten plagues sent by God to force Pharaoh to free the enslaved Hebrews), the Hebrew slaves identified their own homes by marking their doorposts with the blood of a lamb. The angel of death “passed over” those homes so marked, sparing the Hebrew firstborn children. The death of every Egyptian firstborn drove Pharaoh to at last free the slaves, who fled Egypt for Sinai and the Promised Land.

The “main event” during Passover is the Seder (“Order”) meal, which ritually memorializes the above. Matzah, an unleavened bread made without yeast, symbolizes the hurry of the exodus or flight from Egypt, in which there was no time to wait for bread to rise; bitter herbs recall the bitter suffering of the enslaved Hebrews under the Egyptians; a lamb shank stands in for the lamb whose blood marked Hebrew doorposts. The Seder meal is observed on the first two nights of Passover by Jews outside of Israel, and on just the first night alone by Jews within Israel.

I’d like to take this opportunity to wish my Jewish readers around the globe Chag Pesach Sameach (Hebrew for “Happy Passover Holiday” or “Joyous Passover Festival”)!