Even though it’s considered a major holiday on the Jewish calendar, Shavuot — which starts at sundown tonight, May 28 — has always suffered from an image problem of sorts.

Perhaps because of Shavuot’s placement on the calendar around (or shortly before) the official start of summer or due to its lack of home-based rituals, many secular Jews simply don’t know much about it, except that you’re supposed to eat dairy during the holiday.

Adding to the confusion among some folks — even among some fairly informed Jews — is whether Shavuot is observed one or two days.

One or two? Well, what’s it gonna be?

Here’s the deal: If you live in the land of Israel, Shavuot is a one-day holiday. Everywhere else, the festival is celebrated for two days (except by the Reform movement, which keeps only one day). That’s not just Shavuot, either.  

Bear with us, we’re going to get a bit technical here. 

Shavuot, like other Jewish holidays, is lunar — calculated according to the moon. Each month begins on the day of a new moon, when the tiniest crescent of the moon becomes visible after the moon has disappeared altogether. Because the moon cycle is approximately 29½ days long, in some months the next new lunar month begins 29 days after the previous one, and in other months it is 30 days.

Oy.

The many moons among us.
(Photo courtesy of JTA)

Astronomers can now predict the exact day and time of the new moon. But back in antiquity, the date of the new moon could not be calculated in advance — it had to be observed.

Furthermore, according to Halachah, or Jewish law, it had to be “officially” observed. That means the Sanhedrin, the Jewish governing body in the Land of Israel, had to receive sworn testimony from two eyewitnesses and then proclaim a new month.

But what does all of this have to do with observing Shavuot for one day in Israel and two days in the Diaspora?

OK, we’re getting there.

Once the Sanhedrin received testimony of the new moon and declared a new month in the Land of Israel, the message of the new moon was delivered to Jewish communities in the Diaspora so that all Jews would be on synchronized calendars.

According to the Mishnah, the rabbis created a rather clever system of delivering the message about a new month: signal fires atop mountains. The idea was that once a new month was declared, a fire would be lit atop a mountain in Jerusalem, and then another one atop a neighboring mountain, and so forth — spreading the message of the new month quickly across the landscape.

But this system did not work for very long. The Mishnah tells us that the Samaritans, a rival religious sect, sought to disrupt the fire signals by lighting their own fires to confuse the Jews. So in the end, the month had to be declared by horse and rider.

But nu, that could take a long time, and by the time the message got to Diaspora Jewish communities (particularly with regards to holidays that fall in the first day of the month, like Rosh Hashanah), it was too late — the holiday would have already passed!

Thus, the Diaspora communities took to hedging their bets by celebrating major holidays for two days rather than one, just to ensure that they would celebrate it on the same day as their fellow Jews in the Land of Israel.

Today, in honor of that tradition, Jewish communities outside the Land of Israel continue to celebrate major holidays for two days, even though we have the technological capability to synchronize our calendars well in advance.

At any rate, chag sameach! Whether you celebrate one or two days of Shavuot!

This article was provided to JTA by My Jewish Learning (https://www.myjewishlearning.com/).