Arba'ein Chehellom is a Shia religious observance that occurs forty days after the Day of Ashura.
It commemorates the death of one of Shiite Islam's most important leaders, Imam Hussein (Al-Husayn ibn Ali), a grandson of the Prophet Muhammad. Hussein is said to have died 1340 years ago in the dusty plains of Karbala. A grave was established to commemorate his death, and the city of Karbala, in what is now modern-day Iraq, slowly built around it over time.
Every year, millions of pilgrims descend on the central Iraqi city of Karbala, a usually quiet desert city, to commemorate the religious holiday of Arbaeen, one of the largest organized gatherings of people in the world. The events center on two adjacent mosques: the Imam Hussein and Imam Abbas shrines.
What makes Arbaeen unique is the fact that the shrines actively invite leaders and people of other faiths.
source: The New York Times
Tradition holds that, in A.D. 680, Hussein and his followers were on their way to challenge the succession of Caliph Yazid, whom they saw as an illegitimate successor after the death of Prophet Muhammad, the founder of Islam. Yazid responded by sending a massive army to intercept Hussein, who continued to refuse allegiances with the Caliph. A battle ensued, and Hussein and all his followers were massacred. To this day, Hussein's death is a defining drama of the Shiite faith and, in Christ-like fashion, remains powerfully resonant.
On the routes of the pilgrimage, food, accommodation and other services are provided for free by volunteers
The pilgrimage has been described as "an overwhelmingly powerful display of Shia belief and solidarity".
The pilgrims carry flags of different color but the black flag of mourning for Imam Hussein is by far the most common. They also decorate permanent brick buildings and temporary tents which are used for praying, eating and sleeping along the three main routes leading to Kerbala.
Shia Muslims in Iran observe several rituals during the Arbaeen event including, gathering on the streets and marching in long processions and beating their chest, preparing food for the poor, as well as attending Ta'ziyeh which is a kind of street theater in a costume inspired by historical events that happened in Karbala battle.