What Is Hajj?
Hajj is the pilgrimage to Mecca, one of the five pillars of Islam, obligatory once in a lifetime for every adult Muslim who is physically and financially able to make the journey — a condition known as istita'ah (capability), which is why Hajj is not treated as obligatory on everyone equally the way the five daily prayers are.
The pilgrimage takes place during a fixed window in the Islamic calendar, primarily the first ten to thirteen days of Dhul Hijjah, the twelfth and final month of the Hijri year, and involves a specific sequence of rites performed at specific locations in and around Mecca. These include entering the state of ihram (a state of ritual purity marked by specific dress and restrictions) before crossing designated boundary points (miqat); performing tawaf, circling the Kaaba seven times; standing at the plain of Arafat on the ninth of Dhul Hijjah, considered by many scholars to be the single most essential rite of Hajj, to the point that a hadith states "Hajj is Arafat"; and the symbolic stoning of pillars at Mina representing the rejection of temptation, drawn from the story of Prophet Ibrahim.
Hajj differs from Umrah in that it is tied to this specific calendar window and includes rites — most notably the standing at Arafat — that Umrah does not require at all; Umrah can be performed at any time of year and is sometimes called the "lesser pilgrimage" for that reason, though it is a full and independently meaningful act of worship in its own right, not a partial or lesser version of Hajj in terms of spiritual weight.
The financial and physical capability requirement is treated seriously in the fiqh literature — a person is not considered obligated to perform Hajj by taking on debt they can't reasonably repay, or by neglecting the basic financial needs of their dependents, and ill health or lack of a realistic means of travel are recognized as valid reasons the obligation doesn't apply, at least for that year. This is different from framing Hajj as something to be attempted at any cost — the tradition explicitly builds in the capability condition as part of the obligation itself, not as an excuse layered on top of it.