The obligation is held across Sunni and Shia Islam, with only procedural differences among the legal schools, and the lesser pilgrimage, umrah, may be undertaken at any time of year without discharging the once-in-a-lifetime duty. Before entering the sacred precinct, pilgrims adopt ihram, a state of ritual consecration marked outwardly, for men, by two simple seamless white cloths, and governed by restrictions on cutting hair or nails, wearing perfume, hunting, or quarreling, all of which are read as stripping away the ordinary markers of wealth, nationality, and status so that pilgrims stand before God as equals.
The Hajj expresses the unity of the community on a scale unmatched by any other religious gathering, drawing pilgrims from across the world into a single set of rites performed within days of one another, and the logistics of managing that crowd, controlling access to the Kaaba, regulating the stoning at Mina, providing water and shelter across the pilgrimage sites, have become one of the Saudi state's major administrative undertakings, reflecting how a rite rooted in seventh-century Arabia has had to be continually re-engineered for a modern mass pilgrimage of a size the earliest generations could not have anticipated.
As a Pillar it belongs with the fast, the prayer, the alms, and the testimony of faith; its once-in-a-lifetime obligation, conditioned on ability, distinguishes it from pilgrimages other traditions commend but do not require of every adherent.