The ritual of Hajj is performed in the month of ZulHijjah, the twelfth and last month in the Islamic lunar calendar. In Arabic, the word hajj literally means going to a place for the sake of visiting; and according to the terminology of the Islamic Sharee'ah, it implies to the act of going to the Baitullah in Mecca to observe the necessary devotions related to pilgrimage. The hajj is mandatory upon all able Muslims, men and women, at least once in a lifetime.
Apart from worship of Allah (SWT), the command for hajj offers Muslims the opportunity to interact with their brethren from around the globe. The command concerning the hajj is found in several Qur'anic verses, among them:
"And when We assigned to Ibraheem the site of the Sacred House (Ka'bah), saying 'Do not associate with Me anything, and purify My House for those who circumambulate the Ka'bah, and those who stand up and prostrate and bow in prayer. And proclaim the Pilgrimage among men, and they will come to you on foot and mounted on every kind of camel, lean on account of journeys through deep and distant mountain highways." [Surah Al-Hajj: Verse 26-27].
The verse above confirms that Prophet Ibraheem (a.s.) worshipped at the Sacred House of Allah SWT (Baitullah - another name of Ka'bah) long before the advent of Judaism or Christianity.
For that matter, some Bibilical historians say that he lived in the first half of the second millennium B.C, that is prior to the establishment of the Indus civilizations of Harappa and Mohenjo-Daro; hence, prior to the birth of Hinduism. By virtue of it being the fifth pillar of Islam, the hajj literally makes Islam the oldest religion on earth.The hajj today is the largest gathering of human beings on earth.
Besides this annual ritual, there is a lesser hajj known as Umrah that can be performed at any time during the year except on the days of the greater hajj, specifically from 9th till 13th of ZulHijjah.
On the benefit of hajj, Abu Hurairah (r.a.) narrates that he heard the Prophet SAW say:
"Whoever performs Hajj and does not commit any Rafath (obscenity) or Fusooq (transgression), he returns (free from sin) as the day his mother bore him." [Bukhari]
There are three categories of hajj: Tamattu', Qiraan and Ifraad. In brief,
Tamattu' refers to the act of performing umrah in the month of hajj.
Qiraan, on the other hand, refers to a pilgrim who performs hajj and umrah jointly;
Ifraad means that a pilgrim wears the ihraam for hajj only and he/she performs his umrah only after completing the greater hajj.
The most beautiful aspect of hajj is the uniformity. All the pilgrims have to do exactly the same acts at the same sites around Mecca on four days consecutively. All wear the ihraam garb, a great majority of which is white in color.
Once the ihraam is donned and the intention for hajj uttered, male pilgrims are prohibited from wearing stitched clothes, shoes that cover the anklebone, a hat or turban or anything that covers the head. Women are prohibited from wearing the veil that covers their faces (burqaa) and also gloves.
Both men and women are prohibited from wearing any kind of perfume or hair oil, cropping or shaving hair on any part of the body, cut the nails, have sexual intercourse, hunt and kill animals, or to be involved in any form of marriage proposal.