Most of us experience the Jewish calendar in practical terms: When is Passover this year? Why is Rosh Hashanah “late” or “early”? (But never on time?) When do I need to ask for time off? And, of course, why does my phone know all of this better than I do?
But behind those familiar holiday reminders is one of Judaism’s most fascinating systems: a calendar that joins the sun, the moon, rabbinic authority, ancient observation, mathematical ingenuity, and communal life. On Sunday, May 17, at 7 p.m., the Walnut Street Minyan will host “Secrets of the Jewish Calendar,” a visual presentation by astrophysicist Howard Smith exploring how the Jewish calendar works, where it came from, and why it still matters.
“The calendar is what everybody connects through,” Smith said in a recent conversation. It sets the rhythm of Jewish life: fasts and feasts, Shabbatot and festivals, memory and renewal. It is also, he notes, a story of community formation. To share a calendar is to share time. To disagree about a calendar is to raise questions of authority, belonging, and what it means for a people to move through the year together.
The Jewish calendar is neither purely solar, like the secular calendar, nor purely lunar, like the Islamic calendar. It tracks both. Ancient Jewish communities once declared the new month by observing the new moon. Over time, that system developed into a calculated calendar involving leap years, cycles, delays, and rules designed to keep the holidays aligned with both the moon and the seasons.
There are 19-year cycles, 28-year cycles, and far longer patterns. There are rules preventing Yom Kippur from falling immediately before or after Shabbat. There are historical debates over when the fixed calendar began, including the traditional claim that Hillel II established it around 358 CE—a claim that itself has generated centuries of scholarly discussion. In Smith’s words, the calendar is “completely non-random, but totally unpredictable.” In other words: very Jewish. Smith brings an unusual combination of expertise to the subject. A senior astrophysicist at the Harvard & Smithsonian Center for Astrophysics, he is a past member of the Harvard Astronomy Department and served for a decade as chair of astronomy at the National Air & Space Museum. He is also a longtime student of Jewish mysticism and the author of “Let There Be Light: Modern Cosmology and Kabbalah, a New Conversation Between Science and Religion.” His forthcoming illustrated book with Art Green and Debra Band, “Secrets of Creation: Mysticism Meets Science and Art,” will be published this summer by Ben Yehuda Press. For Smith, the calendar opens doors into astronomy, Jewish history, halakhah, philosophy, and the natural world. It reminds us that Sukkot is not only a holiday on a page or screen, but part of an ancient encounter with harvest, equinox, and season. It shows how Jewish life has long balanced precision and flexibility, calculation and observation, heaven and earth. Most of all, Smith hopes people leave with a new sense of wonder. “I want people to come away saying, ‘I didn’t understand most of that, but it certainly gives me a new appreciation, not just for dates and calculations, but for Judaism as a living system of time, meaning, and community.’” There will be munchies and schmoozing. Bring your friends. Your calendar already knows the date; now come find out why.
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