Once each year, Muslims of every ethnic group, color, social status, and culture gather together in Mecca and stand before the Kaaba* praising Allah together. It is a ritual that is designed to promote the bonds of Islamic brotherhood and sisterhood by showing that everyone is equal in the eyes of Allah. It occurs in the month of Dhul Hijjah which is the twelfth month of the Islamic lunar calendar. It is the journey that every sane adult Muslim must undertake at least once in their lifetime if they can afford it and are physically able.
The Hajji or pilgrim wears simple white clothes called Ihram. During the Hajj the Pilgrims perform acts of worship and they renew their sense of purpose in the world. The Hajj makes Muslims feel the real importance of life here on earth and the afterlife, by stripping away all markers of social status, wealth, and pride. In the Hajj all are truly equal.
Mecca is a place that is holy to all Muslims. It is so holy that no non-Muslims are allowed to enter.
*the black cubed structre in Mecca, which Muslims believe Abraham and Ishmael built as a place to worship God.