One of the holiest Muslim holidays — “Eid al-Adha,” the “Festival of Sacrifice” — arrives early next week. In the Islamic religious calendar, based on the moon, the festival falls on the 10th day of Dhu al-Hijjah, the 12th and last month of the Islamic year. However, since lunar years are roughly 11 days shorter than solar years, Dhu al-Hijjah and, therefore, Eid al-Adha — pronounced roughly “eed al-AD-ha” — come about a week and a half earlier annually, according to the familiar Gregorian calendar that is used around the world, including, for secular purposes, by Muslims.

As with the Hebrew calendar (e.g., in Genesis 1), new days begin in the Muslim calendar on the evening of what we would consider “the day before.” This year, for instance, Eid al-Adha begins on the evening of Monday, Aug. 20, and concludes on the evening of Tuesday, Aug. 21— though celebrations may extend somewhat longer.

However, predicting the date of the festival is not entirely routine. While astronomers can reliably predict the moon’s phases, most Muslim authorities insist that Eid al-Adha’s date requires confirmation by an actual, official visual sighting of the crescent moon that commences Dhu al-Hijjah — and, even in sun-drenched Saudi Arabia, weather conditions can sometimes interfere. This year, the festival’s exact date was announced by the Supreme Court of the Kingdom of Saudi Arabia only a few days ago, when a crescent moon had been officially confirmed.

What is Eid al-Adha?

The holiday commemorates Abraham’s obedient willingness to sacrifice his son at God’s command — a story described in the Bible in Genesis 22 and referred to in subsequent Jewish tradition as the “Akedah” or “binding” (of Isaac).

Today, most Muslims believe that the nearly sacrificed boy wasn’t Isaac but, rather, Ishmael, the ancestor of the Arabs. However, the Quran, the holy book of Islam, never actually names the son, and vigorous debate about his identity continued among Muslim scholars for several centuries.

The Quran refers to the story in several places, including 37:100-109, where God speaks using the “royal ‘we’”:

“'My Lord, grant me (a child) from among the righteous!’ So We gave him good news of a patient boy. And when (the boy) had reached the age of working with him, he said, ‘Oh my son! I see in vision that I will sacrifice you. So look! What do you see?’ ‘Oh my father, do what you are commanded, and, if God wills, you will find me among the patient.’

“So, when they had both submitted (their wills) and he had put him face down, We called to him, ‘O Abraham! You have already fulfilled the vision.’

“Thus, truly, do We reward those who do good. For this was clearly a test. And We ransomed him with a great sacrifice, and we left for him, for those afterward, peace upon Abraham.” (our translation)

Ten Dhu al-Hijjah is the third day of the annual Muslim pilgrimage, known as the “Hajj,” when millions of the faithful descend upon the Saudi Arabian city of Mecca. On that day, animals are slaughtered, either by the pilgrims themselves or by agents acting for them, and the meat is distributed among poor people throughout the world.

Prior to those animal sacrifices, though, and again on the following days, pilgrims throw stones at pillars that symbolize Satan. According to Islamic tradition, the devil tempted Abraham during his preparations for the sacrifice of his son, trying to persuade him to disobey the divine command, but Abraham drove Satan away by throwing rocks at him.

However, Eid al-Adha isn’t celebrated only by pilgrims of the Hajj. Throughout the world, on the same day and in solidarity with the pilgrims in Arabia, devout Muslims sacrifice an animal and divide its meat into three parts. The first part of the meat is given to the poor and the second part to relatives, neighbors and friends, while the third part is consumed by the family offering the sacrifice. Muslims with means should also contribute financially for the benefit of the poor. Believers put on their best dress, pray, visit the mosque and exchange gifts with those around them.

When speaking with Muslim friends or co-workers, it is appropriate to wish them a “happy Eid.” In Arabic (and in Muslim countries generally), standard greetings include “Eid Saeed!” (“Happy Eid!”) and “Eid Mubarak” (“Blessed Eid!”). Truly adventurous nonspeakers of Arabic might try “Kullu am wa enta bi-khayr” (roughly, “May every year find you in good health!”).

Daniel Peterson founded BYU's Middle Eastern Texts Initiative, chairs The Interpreter Foundation and blogs on Patheos. William Hamblin is the author of several books on premodern history. They speak only for themselves.