what is chehlum in islam

Chehlum, also known as Arbaeen, is a Shia religious observance that occurs forty days after the Day of Ashura. It commemorates the martyrdom of Al-Husayn ibn Ali, the grandson of Prophet Muhammad, who was martyred on the 10th day of the month of Muharram. Imam Hussain ibn Ali and 71 of his companions were martyred by Ubayd Allah ibn Ziyads army under the governance of Yazid I in the Battle of Karbala in 61 AH (680 CE) . The traditional period of mourning in Islamic culture is forty days, and Chehlum is a part of that mourning period.