The Hajj (Arabic: حج‎ Ḥaǧǧ "pilgrimage") is an annual Islamic pilgrimage to Mecca, and a mandatory religious duty for Muslims which must be carried out at least once in their lifetime by all adult Muslims who are physically and financially capable of undertaking the journey, and can support their family during their absence. It is one of the five pillars of Islam which also includes Shahadah, Salat, Zakat, and Sawm. The gathering during Hajj is considered the largest annual gathering of people in the world. The state of being physically and financially capable of performing the Hajj is called istita'ah, and a Muslim who fulfils this condition is called a mustati. The Hajj is a demonstration of the solidarity of the Muslim people, and their submission to God (Allah). The word Hajj means "to intend a journey" which connotes both the outward act of a journey and the inward act of intentions.
The pilgrimage occurs from the 8th to 12th of Dhu al-Hijjah, the last month of the Islamic calendar. Because the Islamic calendar is a lunar one which is eleven days shorter than the Gregorian calendar, the Gregorian date of Hajj changes from year to year. Ihram is the name given to the special spiritual state in which pilgrims wear two white sheets of seamless cloth and abstain from certain things.
The Hajj is associated with the life of Islamic prophet Muhammad from the 7th century, but the ritual of pilgrimage to Mecca is considered by Muslims to stretch back thousands of years to the time of Abraham. During Hajj, pilgrims join processions of hundreds of thousands of people, who simultaneously converge on Mecca for the week of the Hajj, and perform a series of rituals: each person walks counter-clockwise seven times around the Ka'aba, the cube-shaped building and the direction of prayer for the Muslims, runs back and forth between the hills of Al-Safa and Al-Marwah, drinks from the Zamzam Well, goes to the plains of Mount Arafat to stand in vigil, spends a night in the plain of Muzdalifa, and performs symbolic stoning of the devil by throwing stones at three pillars. The pilgrims then shave their heads, perform a ritual of animal sacrifice, and celebrate the three day global festival of Eid al-Adha.
Pilgrims can also go to Mecca to perform the rituals at other times of the year. This is sometimes called the "lesser pilgrimage", or Umrah. However, even if they choose to perform the Umrah, they are still obligated to perform the Hajj at some other point in their lifetime if they have the means to do so.