Muharram, the first month of the lunar year, is the focus of annual lamentation rituals performed especially by Shi’a Muslims in honor of Hussain b. Ali, the prophet Muhammad's grandson, who was martyred on 10th of Muharram, known as Ashura, in the battle of Karbala. Both in pre-Islamic and Islamic cultures, Muharram is considered a sacred month. It is named Muharram (forbidden) because warfare is forbidden in this month.
Origins in Pre-Islamic CalendarEdit
The name, Muharram, is originally not a proper name but an adjective qualifying Safar. In the pre-Islamic period, the first two months of the old Meccan year were Safar [q.v.] I and II, which is reflected in the dual a potiori of al-Safaran for al-Muharram and Safar; in the old Arab year, the first half year consisted of “Three months of two months each” (the names of each two months were only different in the numbers used in them) (Wellhausen), as the two Safars were followed by two Rabiʿs and two Jumadas.  The pre-Islamic Arabs held the lunar month of Muharram to be sacred during which warfare is forbidden.
Month of MourningEdit
All Muslims consider Muharram to be a sacred period, but it is the Shiʿa who have attached special significance to this month. For the Shiʿa, Muharram is a time to commemorate the martyrdom of the third Imam, Hussain ibn ʿAli, who was killed on the tenth day of the month (Ashura), at the battle of Karbala, Iraq, in 680 CE. The ritualized remembrance of Imam Hussain, his family, and his loyal supporters, who sacrificed their lives for the cause of Islam, extends far beyond Muharram to the months of Safar and al-Rabiʿ al-Awwal. These days of mourning (ayyam-e ʿaza) are a time for the Shiʿa to collectively remember and mourn Imam Hussain’s sacrifice and martyrdom, as well as to publicly affirm their loyalty to the family of the Prophet Muhammad (Ahl-e Bayt) and Islam. Over time, Muharram has come to refer to the collectivity of rituals performed to invoke Imam Hussain’s suffering and sacrifice, as well as to maintain the immediacy of Karbala in the Shiʿi collective conscience.  Muharram is also important in the Sunni tradition, and the ninth and tenth days are days of fasting commemorating when Noah left the ark and when Moses was saved in Egypt. In many parts of the Islamic world, including South Asia and South Africa, Sunnis also participate in Muharram mourning rituals for Imam Hussain and his family, which is considered a way of paying respects to the Prophet Muhammad. Likewise, Muharram has been an occasion for Sunni-Shiʿi violence in places such as Pakistan and Iraq, and for Hindu-Muslim violence in India. 
Mourning Rituals in MuharramEdit
During Muharram, the Shiʿa attend mourning assemblies (majles), where they listen to discourses (rawza- khwani) extolling the idealized qualities (faza’el) and tragic suffering (masa’eb) of Imam Hussain and his family. Memorializing poems of lament are recited (marthiya, salam, and suz), and each majles concludes with the participants beating their chests (Arabic latam; Persian/Urdu matam) in time to rhythmic poems of mourning (nawha). In Iran and South Asia, replicas of Imam Hussain’s tomb (naql, taʿzia) are constructed and carried through the streets in processions (jolus). On 9 and 10 Muharram, men solemnly march through the streets in some regions performing various acts of bloodletting self-flagellation, including striking the head with a sharp knife (tatbir, qameh zani) or striking oneself on the back with chains or blades (shamshir zani, zanjir zani). Since the early 20th century, Shiʿi ulama have debated the impressibility of performing “bloody matam.” In 1994 Ayatollah ʿAli Khamenei issued a fatwa (legal opinion) prohibiting the performance of matam in which weapons are used to shed blood. Likewise, the leader of the Lebanese organization Hezbollah, Ayatollah Muhammad Hussein Fadlallah, has upheld Khamenei’s fatwa, urging Shiʿa to donate blood on Ashura. These legal opinions reflect the desire to deflect criticism away from Shiʿi Muharram rituals, which are often portrayed as excessively violent. Imam Hussain’s martyrdom is dramatically reenacted in Iran, India, Pakistan, Turkey and the Caucasus, Iraq, and Lebanon in the ta’ziya, where village men and professional actors assume the roles of the heroes and villains of Karbala. 
Hadith on MuharramEdit
1: Through successive chain of transmitters, Shaikh Saduq relates from Ibn Masroor, from Ibn Amir, from his uncle, from Ibrahim bin Abi Mahmood, who says that Imam al-Rida said,
Muharram is a month in which bloodshed was considered unlawful by the pre-Islamic pagan Arabs, but our blood was shed in this month. Our sanctity was violated and our children & women-folk were made captives. Our tents were set ablaze and whatever was found therein was looted. And they did not even honor the relation, which we share with the Prophet of Allah. The day on which Imam Hussain was martyred has injured our eyes and our tears are constantly flowing since then. Our dear ones were dishonored on the plains of grief and trials (Karbo bala) making way for sorrows and sufferings until Qiyamah. Thus, sorrowful people should mourn over it (the martyrdom of Imam Hussain), for weeping upon it nullifies the major sins.”
Then he said,
“When the month of Muharram would approach, no one would see my father (Imam Musa al-Kadhim) ever laughing until the tenth, and grief would prevail upon him. And the tenth would be the day of sorrow, grief and lamenting, and he would say: This is the day when Hussain was massacred.”
2: Through my connecting chain of transmitters reaching Shaikh Saduq, who relates from Talqani, from Ahmad Hamadani, from Ali bin Hasan bin Fazzal who relates from his father that Imam Ali al-Rida said,
“Whoever avoids attending to his worldly affairs on the tenth of Muharram, Allah will fulfill all the wishes and desires of this world as well as the hereafter. Whoever considers this day to be a day of mourning, sorrow and weeping for himself, Allah the Glorified will make the day of Qiyamah to be a day of rejoicing for him and his eyes will be cooled in Paradise on account of us. And whoever considers the tenth of Muharram to be a day of prosperity and buys something for his house (considering it a good omen), then Allah will not give him affluence in that thing. And on the day of Qiyamah he will be made to arise along with Yazid,Obayd-allah bin Ziyad and Omar ibn Sa’ad (may Allah’s curse be on all of them) and will be thrown into the lowest abyss of hell.”