The first celebration, Eid al-Fitr, began on Thursday, June 14 this year, the date determined by the Islamic calendar.
The date of Eid al-Adha, said to be the holier of the two Eids, is also dependant on the lunar calendar each year.
Eid al-Adha falls on the tenth day of Dhu al-Hijjah, the 12th and final month in the Islamic calendar.
In the international solar Gregorian calendar, the dates drift about 11 days earlier each year.
The dates of the upcoming Eid al-Adha are determined by the moon sighting, applied on the 29th day of the 11th month of the lunar calendar.
However, the sighting of the new moon will vary between countries, so the exact date depends on local religious authorities.
In Saudi Arabia, the date of Eid al-Adha determines the date of Hajj.
This year, local news has reported that the crescent was sighted, making Sunday, August 12 the first day of the 12th month and August 21 the first day of Eid al-Adha.
What is Hajj?
Hajj is an annual pilgrimage to Mecca in Saudi Arabia, the holiest city for Muslims.
It is a mandatory religious duty for Muslims, that must be carried out one in their lifetimes by all adults who can physically and financially undertake the journey.
It is one of the five pillars of Islam, along with Shahadah (belief in the oneness of God and acceptance of Muhammad as prophet), Salat (prayer), Zakat (charity) and Sawm (fasting).
The Hajj is a demonstration of the solidarity of the Muslim people, and their submission to Allah.
What date is Hajj 2018?
Hajj begins on the eighth day of the Dhu al-Hijjah, so is also dependant on the moon sighting.
This year, it is due to commence on August 19, and will continue until August 24.
On the tenth day then, Eid al-Adha will commence and those on the pilgrimage will observe the festival.
This is the day when an animal will be slaughtered in honour of the prophet Ibrahim.
In Islamic scripture, Ibrahim obeyed God’s order to sacrifice his own son.
But before he could do so, God provided a male goat to sacrifice instead, sparing his son.
In commemoration of this, the slaughtered animal is divided into three parts: one third is given to the poor and needy; another third is given to relatives, friends and neighbours; and the remaining third is kept by the family.
Hajj is intended to be an empowering event in a Muslim’s life, with spiritual merit and the opportunity of self-renewal.
Last year, about two million Muslims travelled to Mecca for Hajj.