Millions of Shias have made their way to Kerbala, Iraq to commemorate 'arbaeen' of Imam Hussein, grandson of the Prophet Mohammed and son of Ali ibn AbiTalib. 
Arbaeen which in Arabic means fortieth is marked on the 20th of Safar, 40 days after Ashura, the day it is believed he and 72 of his male family members and friends were martyred for refusing to accept the oppressive and unjust kingdom of Yazid Ibn Muawiyyah. History states that his womenfolk were then taken captive, including a four year old daughter, shackled, chained, denied their hijabs (head covering) and tortured repeatedly. 
Hussein and his sister Zainab's fight against oppression is revered as one of the greatest sacrifices in the name of Islam, and Muslims especially Shias mourn this 1400 year old incident with great devotion and passion often engaging in azadari, which includes wearing black, recitation of lamentations and sermons, beating of the chest and head, promising to never forget the atrocities and reminding themselves to follow in Hussein's footsteps of standing up to modern day tyrants. 
To read more about the history of Hussein's fight against injustice please visit whoishussain.org