Judaism's holiest day of the year is approaching. What is Yom Kippur? When does it start?

• Yom Kippur, an important Jewish holiday, is approaching.
• Also known as the Jewish Day of Atonement, Yom Kippur offers a time for Jews to reflect on sins or wrongdoings.
• Some Jews fast for the holiday, not eating or drinking for 25 hours.

Yom Kippur, the holiest day of the year in Judaism, is approaching.  

The name of the holiday translates from Hebrew to English as the Day of Atonement, and Jewish people may spend the day fasting, attending synagogue or observing the holiday in other ways. It follows Rosh Hashanah, the Jewish New Year.  

“Spiritually, they say on Rosh Hashanah it is written, and on Yom Kippur it is sealed, the idea being that everything that's going to happen in the year to come, the stage is set during this time between Rosh Hashanah and Yom Kippur,” Danielle Kranjec, associate vice president of Jewish education at Hillel International, told USA TODAY. 

But what is Yom Kippur? When is it this year? Here’s what you need to know.  

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What is Yom Kippur?  

Yom Kippur, or the Day of Atonement, is when Jews reflect on sins or wrongdoings from the previous year. Many Jews will attend services at synagogues or other congregations, reciting special prayers and singing special songs.  

“During the course of the year, people get off track,” Steven T. Katz, the Slater professor of Jewish and Holocaust studies at Boston University, told USA TODAY. “They don't keep their obligations. They don't follow the law. They mistreat their neighbors. They are egotistical and self interested. So the object is to try to reassert a kind of camaraderie, reassert a harmonious and ethical relationship between human beings and also between the world above and the world below.”  

“It represents the moment that is established for reorienting ourselves in the right direction. No other festival has quite the same spiritual power as the idea of Yom Kippur,” he added. 

Some Jewish people may apologize to friends and loved ones too. 

“A deeply Jewish idea is that if you’ve harmed another person, only that person can forgive you,” Kranjec said. “Many people in the time between Rosh Hashanah and Yom Kippur try to take stock of their relationships and directly ask forgiveness for harm that they’ve caused to another person.”  

When is Yom Kippur? How long does it last?  

Yom Kippur begins at sundown on Tuesday, Oct. 4, and ends in the evening on Wednesday, Oct. 5. It lasts one day, while Rosh Hashanah lasts two days.  

Why do Jewish people fast on Yom Kippur?  

One of the most common Yom Kippur traditions is to fast for 25 hours, not eating or drinking from the night Yom Kippur begins into the night it ends.

Kranjec explained that “Many Jewish rituals and observances have to do with sensory experiences, whether that's enjoying delicious food that's then elevated with a blessing or another kind of moment of mindfulness.”    

“Yom Kippur is really the one day on the Jewish calendar when Jewish people attempt to transcend the physical limitations of being in a human body,” she added.  

Some Jewish people also avoid other actions on Yom Kippur, such as bathing, applying makeup, wearing leather shoes or having sex.  

“These things have a kind of sense of limiting the ego. When you're fasting, you don't feel quite so powerful. You don't feel you're in charge. You don't feel that you're in control. When you don't wear leather shoes, it's a sign, again, of withdrawal,” Katz said.    

How else is Yom Kippur observed?  

Many Jewish families and communities will gather before Yom Kippur begins and after it ends to have festive meals, to prepare to fast and then to break their fasts together.  

Another important observance is the blowing of the shofar, or a curved ram’s horn. The shofar is sounded ceremonially to conclude Yom Kippur, Kranjec said.  

“That is an important communal moment where the closing prayers of Yom Kippur are said together, and someone blows the ram's horn and everyone hears it together, and then the fast is broken together,” she said.  

Is it appropriate to say Happy Yom Kippur?  

No, saying “happy Yom Kippur” to your Jewish loved ones doesn’t strike quite the right tone, since the holiday isn’t typically a joyous one.  

“In English, you might say to friends or colleagues ‘have a meaningful Yom Kippur,’ Kranjec said. “Focusing on the meaning of the holiday and saying to people ‘have a meaningful fast if you're fasting,’ or you could even say have a good Yom Kippur, but happy is probably not the right adjective.”