Millions of Muslims from around the world have gathered in the Saudi city of Mecca in anticipation of the start of Hajj - the annual Muslim pilgrimage.

　　Taking part in the pilgrimage at least once in one's lifetime is a major obligation for all able-bodied Muslims of financial means, and between two and three million people participate in the six-day ritual every year.

　　Hajj occurs in the 12th month of the Islamic lunar calendar, called Thul-Hijjah, between the eighth and the 13th days of the month.

　　Hajj is, put simply, complex. There are several different ways of performing it.

　　The explanations that follow are, thus, a simplification of the pilgrimage, meant to lay things out clearly, rather than comprehensively.

　　Start of Hajj, 8th of Thul-Hijjah

　　The very first rite of Hajj is entering ihram - a pilgrim's sacred state - when crossing the outer boundaries of Mecca, called Miqat.

　　Pilgrims have been arriving in Saudi Arabia for weeks in anticipation of this day. On the eighth of Thul-Hijjah, pilgrims enter ihram, which entails wearing plain garments - two unstitched clothes for men, or any loose-fitting clothing for women - as well as following certain rules, including not hunting or engaging in sexual activity.

　　They then set out from Mecca en masse to Mina, whether by foot along pilgrim paths, or by buses and cars. It is an 8km journey.

　　The pilgrims will spend the day in Mina, only setting out the next morning at dawn.

　　Day of Arafat, 9th of Thul-Hijjah

　　The Day of Arafat is considered one of the most important days, not just of Hajj, but of the Islamic calendar. Mount Arafat was the scene of the Prophet Muhammad's, may Allah exalt his mention, final sermon, and after making the 14.4km journey from Mina, pilgrims spend the day here in reverent prayer.

　　Elsewhere in the world, many Muslims choose to fast on this day.

　　After sunset, its time to move again, this time to Muzdalifah - a 9km trip - where they spend the night under the stars. Many will also begin collecting pebbles here for tomorrow's rites, departing again just before sunrise.

　　'Eid day,10th of Thul-Hijjah

　　The 10th of Thul-Hijjah is 'Eid al-Adh'haa, a day celebrated by Muslims around the world as the greater of the two Muslim holidays.

　　For those performing Hajj, the day is know as Yawm al-Hajj al-Akbar (The big Hajj day) and is probably the longest day of the pilgrimage.

　　Pilgrims start the day in Muzdalifah and begin heading back to Mina before dawn. Once in Mina, they perform the first Ramy, throwing pebbles at columns known as Jamarat.

　　Millions of pilgrims converge at the Jamarat bridge, which houses the three columns representing the devil.

　　The bridge has been the sight of deadly stampedes in the past, with around 350 people being crushed to death in 2006.

　　But in recent years, the event has taken place without major incident, and this year, Saudi Arabia has unveiled an expanded five-storey structure to accommodate the crowds.

　　After casting their stones, pilgrims perform the sacrifice.

　　Pilgrims thus slaughter a sheep, goat, cow or camel - or more likely, pay for it to be done in their names.

　　At this point, pilgrims trim or shave (men only) their hair and remove their ihram clothes. Many will then proceed to Mecca to perform Tawaf and Sa'y, first circling the Kaaba seven times, then walking seven times between the hills of Safa and Marwa.

　　When all is finally done, they return to their campsite in Mina.

　　Final days,11th-13th of Thul-Hijjah

　　With the hardest part behind them, pilgrims will now spend the next two or three days in Mina.

　　On each day, they will again perform throwing at each of the three pillars.

　　When their time in Mina is finished, pilgrims return to Mecca to perform the final circulation of the Kaaba, a "farewell" tawaf.

　　Before heading home, many also go to Medina, the second holiest city in Islam, where the Prophet Muhammad, may Allah exalt his mention, is buried along with his closest companions, may Allah be pleased with them. Visiting Medina, however, is not a part of Hajj.