Arbaeen means forty. The observance falls on the fortieth day after Ashura, the killing of Husayn ibn Ali and his companions at Karbala in 680, and forty days is the traditional term of mourning in the region, the point at which a household would ordinarily close its formal grief. The claim implicit in Arbaeen is that this grief does not close.
The tradition holds that Jabir ibn Abdullah al Ansari, a companion of the Prophet by then old and blind, came to the grave on that fortieth day, making him the first to perform this visitation, and a specific text, the Ziyarat Arbaeen, is recited at the shrine.
The walk from Najaf to Karbala is about eighty kilometres, marked with numbered poles counting down, and pilgrims also come from Basra and from the Iranian border on far longer roads. It is sustained by mawakib, the roadside stations that feed, treat, wash and shelter walkers without charge, run by families, villages, unions and businesses who regard providing the service as their own participation.
The political history matters. Public Shia mourning processions were restricted and then banned under Baathist rule, and the attempt to hold them was repeatedly met with force, including the suppression of the Safar disturbances in 1977. Walking the road was a prosecutable act for a generation. The observance resumed after 2003, at once and at enormous scale, and its size is partly an assertion about that suppression.
The published attendance figures run into the tens of millions and come from the Karbala authorities and the shrine administrations. They are not independently audited and should be read as the organisers' own count, which is how this entry reports them.