Why do 20 million humans from all around the globe gather on the land of Karbala, Iraq?

TEHRAN(Basiart):Millions are making their way to Iraq's Holy city of Karbala to celebrate the birth anniversary of the third Shia Imam, Imam Hussain. He is revered by many Muslims and non-Muslims alike as a 7th-century revolutionary figure who stood against injustice and oppression which plagued the Muslim nation under the corrupt rule of the Umayyed Caliphate.