Muḥarram al-Ḥarām (Arabic: مُحَرَّمُ الحَرام) is the first month of Hijri calendar and one of the four Ḥarām months in Islam and Pre-Islamic Arabian culture. It is called "Muharram" (forbidden) because fighting is forbidden in this month. It is called "Muharram" (forbidden) because fighting is forbidden in this month. The importance of this month for Shi'a is because the event of Ashūrā and the martyrdom of Imam al-Ḥusayn (a) and his companions took place in this month. Every year, Shi'a commemorates the martyrdom of their Imam (a) in this month by holding mourning sessions.
After martyrdom of Imam al-Ḥusayn (a) in this month, Muharram turned to the month of mourning for Shi'a. From the earlier years after Imam's (a) martyrdom, the following infallible Imams (a) and Shi'a held mourning sessions in this month. For instance, Imam al-Rida (a) said, "When the month of Muharram arrived, no one would see my father laughing. Grief and sorrow was apparent on his face to the day of 'Ashura'. That day ('Ashura') was the day of his grief, sorrow and weeping. He said, "Today is the day that Ḥusayn (a) was martyred."
Imam al-Ridā (the eighth Imam) says, “The month of Muharram is one of the months during which the people of pre-Islamic (pagans) prohibited fighting. But (in the same sacred month) the people considered it lawful to shed our blood. (In this month) our honor was violated, our progeny and women were made captive, fire was put in our tents, our belongings there were plundered, and no inviolable thing that the Messenger of Allah had in us remained respected. Verily, the Day of al-Hussain made our eyes wounded and let flow our tears and humiliated our beloved in the land of Karbalā, the land of tragedy and affliction that just brought to us sorrow and difficulties till the Last Day. So the weepers should weep over the like of al-Hussain. Indeed, weeping for al-Hussain brings about forgiveness for great sins.” (Bihār al-Anwār, vol. 44, p. 283) Mirza Jawad Maliki Tabrizi writes in his book al-Muraqibat asys: “In this month and especially in the first ten days, devotees of the Prophet's household (a) should be sad in their heart and appearance. They also should leave some Halal enjoyments, especially in 9th, 10th and 11th of this month, like someone bereaved. In the first ten days, they should remember their Imam (a) by reading Ziyarat al-'Ashura'.”