The Arba'in pilgrimage is the world's largest annual public gathering. It is a pilgrimage to the shrine of Husayn ibn Ali, grandson of the Islamic prophet Muhammad and the third Shia imam. Every year, on the twentieth of Safar, also known as Arba'in, millions of pilgrims flock to Karbala, Iraq, often arriving there on foot from the nearby city of Najaf. Arba'in marks forty days after the tenth of Muharram, known as Ashura. On this day in 61 AH (680 CE), Husayn was killed, alongside most of his relatives and his small retinue, in the Battle of Karbala against the army of the Umayyad caliph Yazid ibn Mu'awiya (r. 680–683). The battle followed Husayn's refusal to pledge his allegiance to Yazid, who is often portrayed by Muslim historians as impious and immoral. In Shia Islam, Karbala symbolizes the eternal struggle between good and evil, the pinnacle of self-sacrifice, and the ultimate sabotage of Muhammad's prophetic mission.