(CNN)From Sunday, more than a billion Muslims around the world will celebrate Eid al-Adha, which translates to the Festival of Sacrifice.
Here's what you need to know about Eid al-Adha, one of Islam's biggest holidays
One of the most important holidays of the Islamic calendar, Eid al-Adha marks the height of the annual Hajj pilgrimage to Mecca in Saudi Arabia. It commemorates the story in the Quran of God appearing to Ibrahim -- also known as Abraham -- in a dream and commanding him to sacrifice his son as an act of obedience.
Muslims believe that as Ibrahim was about to sacrifice his son, God stopped his hand and gave him a sheep or ram in place of his son. A version of the story also appears in the Book of Genesis in the Old Testament and in the Torah.
To commemorate God's test of Ibrahim, many Muslim families have an animal slaughtered -- often through their local butcher -- and distribute the meat to family, neighbors and the poor.
It is a time of celebration when families gather for meals, visit relatives and friends, and children are given money and new clothes. It's also a time when families visit the graves of their loved ones and donate to the poor.
The day begins with morning prayers. The celebrations continue with visits to friends and family, exchanges of gifts and feasts.
Eid Mubarak (pronounced EED muh-BAR-ack) and Eid Saeed are routine greetings used during the observance to offer best wishes.
The four-day religious holiday begins on the third day of the annual Hajj, the pilgrimage to Mecca that all Muslims who are physically and financially able are required to undertake once in their lifetime -- some saving their whole lives to make the journey.
Because the timing of the festival depends on the sighting of the new moon, countries around the world sometimes celebrate it on different days, though most follow Saudi Arabia's lead as it is the host of the Hajj.
This year, it will start Sunday and end Thursday.