Every year Muslims celebrate two feasts. Because the Muslim world is very large, with Muslim majority countries stretching from Morocco in the West to Indonesia in the East, with adherents and believers coming from a wide variety of backgrounds, practices during and even names of this feast differ from one place to another. One feast happens at the end of Ramadan, the month of fasting, and is called the Feast of Breaking the Ramadan Fast (ʿīd al-fīṭr; عيد الفطر). Or more informally it could be translated as the Breakfast Feast--afterall, breakfast is all about breaking your fast from the night, usually while you've been sleeping. The other feast takes place almost two and a half months later, and is called the Feast of the Sacrifice (ʿīd al-aḍḥa; عيد الاضحى). It refers to the sacrifice Abraham is recorded to have made of his son. In some places in the Muslim world this is referred to as the Great Feast or the Big Feast, differing it from the lesser feast at the end of Ramadan.
However the Feast of Sacrifice is a celebration which takes place within a wider event of great significance in the life of a Muslim--the Hajj (ḥajj; حجّ). The Hajj is a spiritual pilgrimage that Muslims are required to do at least once in their lifetime, provided that they have the material and physical ability to do so. This pilgrimage should be taken to the holy city of Mecca, and it should be done during a specific time of the year, beginning on the tenth day of the month of Dhū al-Ḥijja (translated as the month of the pilgrimage). As the Muslim calendar moves forward ten or eleven days every year in comparison with the Gregorian calendar, the date of the Hajj changes from year to year and through the seasons. This year, 2018, the 10th month of the pilgrimage is on August 21. In preparation for the feast day, all those who are not on pilgrimage will be preparing to make some form of sacrifice in order to commemorate what Abraham did in order to honor God through his obedience.
The concept of sacrifice is ancient, dating from before the time that the God of Abraham called him to leave his family's region and go a place God would reveal to him. In fact, the sacrificing of children, like the command understood to have been given to Abraham, was a practice that pagans also did to gods that they recognized. In the narrative related to Abraham, whether you read it in the Old Testament or in the Qurʾan, it is understood that Abraham did not end up sacrificing his son, but in his obedience, and preparedness to do that, God stopped him and provided an animal to be sacrificed in his place. The narrative in the Qurʾan provides only some details and it has been debated through history as to who this son was, as he isn't specifically named in the text. Popular belief today among Muslims says that it was Ishmael. Regardless of which son it was that was to be sacrificed, the message is clear in either text and in all three faith traditions, Judaism, Christianity and Islam--God provides.
Beginning tomorrow, many Muslims from around the world will be beginning their pilgrimage to the city of Mecca, the location believed within Islam to be home of the first house of worship to the one true God. A few days later, on August 21, all Muslims around the world will celebrate the sacrifice Abraham made of an animal, in place of his son. In commemoration, many Muslims will sacrifice an animal and share that meat with family, friends, and even with the poor. If you see a Muslim, you can greet with with either an English or Arabic greeting--Happy Feast, or ʿīd mubārak. May God provide for us all.