The Day of Arbaeen marks forty days after the Day of Ashura, the day Hussain ibn Ali "AS" was killed in the Battle of Karbala. Hussain ibn Ali was a 7th century revolutionary leader who sacrificed his life for social justice.
The Arabic word ‘Arbaeen’ (pronounced Arr-ba-een) translates to forty (ie fortieth day). The traditional period of mourning in Islamic culture is forty days.
As such, millions of people around the world mark the Day of Arbaeen by remembering the story of Hussain, his family, and his companions.