Arba'een ("forty"), Chehlom ("the fortieth [day]") ("the fortieth of Imam") is a Shia Muslim religious observance that occurs forty days after the Day of Ashura.
It commemorates the martyrdom of Husayn ibn Ali, the grandson of Muhammad, which falls on the 10th day of the month of Muharram. Imam Husayn ibn Ali and 72 companions were killed by Yazid I's army in the Battle of Karbala in 61 AH (680 CE). Writing in forty batches has become a tradition among Islamic scientists. The reason is the famous saying of the prophet of Islam: "On the day of judgment, among my people, God will consider whoever memorized forty Hadiths as an erudite man". In this regard, numerous Islamic scholars have gathered precious collections that each consists of forty sayings, Hadiths, quoted from the prophet and the following Imams. According to al-Jizzini,The primary martyr, he, too, has written his book Al-Arbaʿūn ḥadīth to practice this narrative of the prophet. Arba'een or forty days is also the usual length of mourning after the death of a family member or loved one in many Muslim traditions. Arba'een is one of the largest pilgrimage gatherings on Earth, in which up to 30 million people go to the city of Karbala in Iraq.