Yom Kippur, the Day of Atonement. is widely considered the holiest day of the year in the Jewish calendar. It is being observed this year from around sunset on Sunday until after nightfall Monday night. It is typically spent in prayer and repentance.

It marks the conclusion of the Jewish High Holy days, which began with Rosh Hashana, the Jewish new year, which began and ended last week.

Brief descriptions of these holy days are available here from two of the major branches of U.S. Judaism:

—The Hasidic organization Chabad-Lubavitch:

Rosh Hashana

Yom Kippur

—The Union of Reform Judaism:

Rosh Hashana

Yom Kippur

In recent years, many synagogues around the world have added extra layers of security, particularly on the High Holy Days. These include guards, cameras, and various systems for controlling access to events through ticketing, registration or other forms of vetting.

“The security measures that congregations have to upgrade to is just overwhelming,” said Rabbi Rick Jacobs, leader of the U.S.-based Union for Reform Judaism. “The feeling of being in community — we desperately need that. But you don’t want to tell people something that’s false, like ′Don’t worry.′”

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