4 Things to Know About Yom Kippur

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Yom Kippur is a Jewish holiday that is marked by its solemnity and a day to begin the new year with a fresh start.

Yom Kippur, which begins on Sept. 18 this year, is one of the most important religious days in Judaism. (Though it might not seem that way based on U.S. popular culture, Hanukkah is actually one of the least important.) The holiday is inseparable from its 25-hour fast, which also prohibits drinking water — even the water used for brushing one’s teeth.

Here’s what you should know about Yom Kippur and what it means for those observing.

What is Yom Kippur?

Yom Kippur is a Jewish holiday that translates to “Day of Atonement” in English. It follows Rosh Hashanah, the Jewish new year, by a little more than a week — and the two are known as the “Jewish high holidays” or “high holy days.”

Rosh Hashanah, which began on Sept. 9 this year, marks the beginning of the Hebrew calendar — currently in the year 5779 — and is considered a time for joy and celebration. When Yom Kippur takes place shortly thereafter, it is supposed to be a time for observing Jews to repent for their sins and heal their souls for the new year.

Yom Kippur is the “holiest day of the year” for Jews, according to Chabad, a worldwide Orthodox Jewish movement.

When is Yom Kippur?

Yom Kippur begins at sundown on Tuesday, Sept. 18, and ends at sundown on Wednesday, Sept. 19.

The fast lasts for 25 hours, rather than the typical 24 of a full day. The 25-hour observance on Yom Kippur allows a cushion of time for the subjectivity of “nightfall” as a moment in time, Chabad says. Those familiar with the Jewish faith may know that Shabbat, or the sabbath, lasts for 25 hours, too.

Why do Jewish people fast on Yom Kippur?

Yom Kippur is characterized by a 25-hour fast. The timing of this fast differs from fasting during the Muslim month of Ramadan, when food can be consumed after sundown each night and before sunrise each day.

For Yom Kippur, which only occurs for one day, the fast is observed from sundown to sundown. By extending the holiday for an extra hour, those observing can be sure that the fast aligns properly with the date.

The fast occurs as part of the somber mood of the day of repentance. However, as with all Jewish holidays marked by a fast (there are six in total), those with medical conditions are not expected to put themselves in danger by fasting. “The same Torah which commands us to fast on Yom Kippur tells us that guarding our health is far more important than fasting on this holy day,” Chabad says.

What should you say to someone observing Yom Kippur?

On Yom Kippur, it is customary to say “have an easy fast,” rather than something jovial like “happy holidays.” Those observing often say “yom tov” in Hebrew or “good yuntif” in Yiddish, both of which are a wish to “have a good holy day.”

But if you say “happy holidays,” don’t worry — the holiday will become happy later that evening with a meal to break the fast after sundown. The “break-fast,” not to be confused with its homonym “breakfast,” is the traditional meal following a fast. (Being the first meal of the day, a daily breakfast serves the same purpose.)

Jews from all over the world break their fasts after Yom Kippur with their own traditional fare, but some customary foods served at a break-fast meal include dairy products and light foods, so as not to shock the system after 25 hours with no sustenance.

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Write to Rachel E. Greenspan at rachel.greenspan@time.com