It’s not the Muslim Hajj, or the Hindu Kumbh Mela.. Known as Arbaeen, it is the world’s most populous gathering and you’ve probably never heard of it! Video:World’s Biggest Pilgrimage Known as ArbaeenREAD MORE:World’s Biggest Pilgrimage, And Why You’ve Never Heard of it! By Sayed Mahdi al-Modarresi, first published in The Hoffington Post.