Arba'in, observed on the 20th day of the lunar month of Safar, marks the 40th day after the martyrdom of Imam Hussein, the grandson of Prophet Muhammad and the third Imam in Shia Islam. On this day, Shiites embark on a pilgrimage from Najaf to Karbala, the burial site of Imam Hussein, which is considered sacred by Shiites. In recent years, this event has attracted a substantial number of participants, with estimates ranging from 15 to 18 million, making it one of the largest religious gatherings in the world.