It is incumbent upon every Muslim who can afford to do so to make the pilgrimage to “the Ancient House” (the Ka’ba) at Mecca at least once in his or her life.Those who cannot afford to do so or are otherwise unable are exempt from the requirement.
Make the pilgrimage and visit the Sacred House for His sake. If you cannot, send such offerings as you can afford… Make the pilgrimage in the appointed months. He that intends to perform it in those months must abstain from sexual intercourse, obscene language, and acrimonious disputes while on pilgrimage … Provide [for] yourselves well: the best provision is piety.
What this means, in practice, is that for a few days each year the population of Mecca swells by tens and hundreds of thousands. Muslims from around the world gather and dress in white to perform the complex multi-day rituals of the hajj. In premodern times, this was a remarkable opportunity for Muslims from Morocco to meet and share ideas with Muslims from Indonesia and Kenya. Many a coup was planned there, as well. It is a time of great international solidarity, when Muslims are reminded of the worldwide character of their faith and of the strength of their numbers. (In this regard, it somewhat resembles the annual general conference of the Latter-day Saints.)
Some of the rituals seem to be of astronomical origin. Others commemorate historical events such as the travails of Hagar and Ishmael in the desert. Unfortunately, for many years non-Muslims have been legally prohibited from entering either Mecca or Medina—on pain of death. Nevertheless, the rituals are not secret, and photography is allowed, so non-Muslim scholars in the West actually know a great deal about the hajj although we ourselves cannot observe it in person.
 The intrepid nineteenth-century explorer Sir Richard Francis Burton managed to sneak into Mecca and Medina and to make the pilgrimage. He published a two-volume Personal Narrative of a Pilgrimage to al-Madinah & Meccah which is still available in paperback reprint from Dover Books. He was so courageous, in fact, that he even undertook an expedition to Salt Lake City in 1860, where, among other things, he met Brigham Young. In 1861, he published a book entitled The City of the Saints and Across the Rocky Mountains to California.