Hajj is the Islamic pillar requiring a physically and financially able Muslim to make a pilgrimage once.
The Five Pillars are the core practices commonly identified in Sunni and Shia Islamic teaching: shahada, salat, zakat, sawm, and hajj. Hajj is unusual among them because its obligation is explicitly conditioned on ability, often described by the Arabic term istita‘ah. Muslims who cannot safely or reasonably undertake it are not expected to perform it.
The pilgrimage is held in Mecca during Dhu al-Hijjah and includes several rites over designated days. Its communal scale brings Muslims from many countries together in shared clothing, movement, prayer, and ritual. The pilgrimage also concludes around Eid al-Adha, the Festival of Sacrifice.
Hajj is not the same as umrah, which is a voluntary pilgrimage available at other times. Nor is simply visiting Mecca automatically hajj: the specific rites and timing are essential to fulfilling the pillar.