Introduction to Haj

The Hajj (Arabic: حج‎) is a pilgrimage to Mecca (Makkah). It is the largest annual pilgrimage in the world.[1] It is the fifth pillar of Islam, an obligation that must be carried out at least once in their lifetime by every able-bodied Muslim who can afford to do so. It is a demonstration of the solidarity of the Muslim people, and their submission to Allah.[2] The pilgrimage occurs from the 8th to 12th day of Dhu al-Hijjah, the 12th month of the Islamic calendar. Because the Islamic calendar is a lunar calendar, eleven days shorter than the Gregorian calendar used in the Western world, the Gregorian date of the Hajj is eleven days earlier from year to year. In 2007, the Hajj was from December 17 to December 21; in 2008 from the first week of December. The Hajj is associated with the life of Muhammad, but the ritual of pilgrimage to Mecca is considered by Muslims to stretch back to the time of Ibrahim and Isma'il, prominent figures in both Islam and in Judaism. 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. As part of the Hajj, each person walks counter-clockwise seven times about the Kaaba, the cube-shaped building which acts as the Muslim direction of prayer (qibla); 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; and throws stones in a ritual Stoning of the Devil. The pilgrims then shave their heads, perform an animal sacrifice, and celebrate the four day global festival of Eid al-Adha.[3][4][5]
As of 2006, an estimated three million pilgrims participated in this annual pilgrimage.[6] Crowd-control techniques have become critical, and because of the large numbers of people, many of the rituals have become more stylized. It is not necessary to kiss the Black Stone, but merely to point at it on each circuit around the Kaaba. Throwing pebbles was done at large pillars, which for safety reasons were in 2004 changed to long walls with catch basins below to catch the stones. The slaughter of an animal can be done either personally, or by appointing someone else to do it, and so forth.[7] But even with the crowd control techniques, there are still many incidents during the Hajj, as pilgrims are trampled in a crush, or ramps collapse under the weight of the many visitors, causing hundreds of deaths. [8]
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 perform the Umrah, they are still obligated to perform the Hajj at some other point in their lifetimes.

Reference; wikipedia