At sunset yesterday (Tuesday, May 14), Jews worldwide began the Jewish holiday festival of Shavuot (pronounced “shav-oo-OT”), otherwise known as the Feast of Weeks.

Shavout is traditionally both an agricultural harvest festival (originally dating back to when the first fruits of the harvest season were brought to the Temple in Jerusalem as sacrificial offerings before the God of Israel), as well as a later commemoration of the giving of the Torah (Teaching, Instruction, Law) from God to Moses and to Israel as a whole at Mount Sinai, following the exodus of the ancient Israelites from slavery in Egypt.

The Hebrew term shavuot literally means “weeks,” and refers to the fact that the Shavuot festival occurs seven weeks after Passover. The Jewish holiday of Pesach (or Passover) celebrates the exodus, while Shavuot celebrates the subsequent receipt of the Torah.

Why does Shavuot, like all Jewish holidays, begin and end at sunset? Judaism traditionally reckons a “day” as technically beginning not at 12:00 midnight, nor at dawn, but instead at sunset. Shavuot therefore began at sunset yesterday (May 14). It is traditionally observed by Jews in Israel, and by Reform Jews everywhere, for one day (concluding at sunset today, May 15), but by other Jews outside of Israel for two days (concluding therefore at sunset tomorrow, May 16).

Shavuot always begins on the 6th day of the Hebrew month of Sivan. However, that fixed starting date of 6 Sivan on the Jewish religious calendar does not always coincide with May 14 on the secular Western (or “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, Shavuot (always 6 Sivan on the Hebrew calendar) began at sunset on May 26, 2012. Next year, by contrast, Shavuot will begin at sunset on June 3, 2014.

Some Jewish holidays are regarded as “working” holidays (work being permitted upon them), whereas other Jewish holidays are considered to be “non-working” holidays (during which work should be avoided; in a few cases, work may be permitted but only with certain restrictions).

For instance, the Jewish holiday of Chanukah (or Hanukkah), familiar to many non-Jews because of its proximity to the Thanksgiving and Christmas holiday season, is a working holiday. Chanukah runs for eight days, during which time work is permitted, except on the weekly Shabbat (Sabbath) which falls within that eight-day period.

Shavuot, by contrast, is a non-working holiday, meaning that no work (that is, no “work” as defined by the Torah) is permitted during its duration.

In addition to avoiding work and partaking in festive holiday meals and synagogue prayer services, traditional Shavuot observance typically also involves reading special liturgical poetry, eating special dairy foods, overnight Torah study, and a reading of the Book of Ruth.

So, to all of my Jewish friends and readers around the globe, may I today wish you a Chag Sameach (Hebrew for “Happy Holiday”)!