10th Muharram is the biggest day ever for Shiite religion, more so important than the two holy Eids, more important than prayer, more important than pilgrmige, it is the most important day ever. The day Hussein was killed in Karbala, some 1400 years ago. Here are some widespread practices during this day:
Maseer (March) Shiite pilgrims walk from everywhere towards Karbala, in a symbolic cememoration of Hussein's march towards that accursed city with 74 of his family and women and children(why?) to meet the Umayyad army. While this practice was present in individual patterns, it did not become widely adopted until 1970.
Flagellation At its most extreme, Shi'ism rituals can be as severe and ominous as any Opus Dei around - Flagellation or Latum is the beating of chests in unison to the rhythm of a sung verse by a radood, a person who recites a poem usually about the death of a Shiite religious figure, be it Ali, Fatima, Hussein, Zainab, Abbas and even recently, the destruction of the reverred Askari shrines in Samarra. The beating of chest is a traditional Arab way of showing grief, mostly performed by women.
Tatbeer or (head-splitting) This practice is derived from the way Imam Ali was killed, while he was praying dawn when a person called Bin Muljim killed him by a sword which hit his forehead. As a form of solidarity and remembrance, a mutabir brings a sword to his forehead and gently stabs with the wide sheath of the sword - blood usually spurts in great lengths, eventually covering the whole person in blood, it is one of the most controversial and sickening practices, and has been introduced by a Turkish brigade coming from the Kaukaz.
Tashbeeh or Re-enactment Those are convoys which re-enact the battle of al-Taff with great detail, from the beginning of the battle until the decapitation of Imam Hussein. This was imported from Iran at the end of the 18th century.
All these practices above are considered the most iconic, graphic representations of Shi'ism today. In particular the flagellation, Most religious Shia scholars condemn all these acts, but all their edicts against them have been met with disapproval and rejection, and sometimes even anger. Religion sometimes opposes thinking, and as the majority of Shiites in Iraq are common and poor, they oppose rationalization and follow only emotion, and consider this as an unthinkable practice that may be for some people the most important, and basically the nutshell, of religion, to try and shed the most tears in hope of getting the intercession of the holy figure they are mourning. The scholars had to grudgingly accept this, a scholar once said: 'The truth of Islam has been lost between the Sunni and Shi'ite scholars, Sunnis followed the whims of the ruler, and Shiites followed the whims of the people." There are also other reasons for this complete disobedience, The Ashura ceremonies is a massive group-activity that invokes feelings of strength and identity, it is the only purely Shia activity to be practiced on such a massive scale, and anything against it will fall on deaf ears. This is why Shiite political factions greatly pushed this exercise after post-Saddam war, despite the unfortuante result of so many people being exposed as an easy prey for any would-be malignant force, despite repeated terrorist attacks which killed many people in 2003 and 2004, Shiites politicians have stubbornly pushed people to practice them, as if Shi'ism is all about this - a foolish show of defiance. Also, a whole trade has been created around these practices, it is interesting to notice that most of the people who actually perform those flagellations and tatbeers are from the dregs of society and the lowest of lows, and have nothing to do whatsoever with religion. It's more about identity and the ominous flair of the processions.
Today, religion is all about big show, most of Iraqis (and Arabs) are not traditional Muslims in the sense, most of them think about sex more often than God.
Admittedly, those processions are somewhat exotic in their own way, any serious fan of violent, angry music should give Shia latmiyas a go, once you get past all that initial disgust and bypass all religious identification, try to look at it as you would look at Greek mythologies or read the Odysseus. It's got plenty of cool stuff: a plethora of sad, dark melodies ; hordes of epic battles between Good and Evil, with Evil always victorious - and also, it sends shivers down one's spine when you remember that the only musical instrument being used, the oddly time-signatured precussion is made by thousand hands beating down a thousand chests. Look at this frightening verse from a latmiya about the death of Imam Ali:
and you are Death, on the day of Death, when Death
obeys your every wish and command.
Everyone who would hear of your tales
would tremble in fear, of your presence
The most famous Shia performer is undoubtedly superstar Bassim al-Karbalie, I think he is one of the best performers I've ever heard - his poems greatly use a tool which Western Music could only dream of having: the quarter-note.
Blood, violence, darkness, epic battles, long-hidden secrets. Screw Maiden, man! Of course, it's kinda frightening that some people actually believe this shit, but then again, what more could you possibly want?