Participating in the mourning of Imam Hussein (peace be upon him).

The commemoration of the Ahlulbait (Peace be upon them) is amongst the best deeds that the religious texts have emphasized on, and it is narrated that there are great rewards for those who perform it.

What the lovers of the Ahlulbait (peace be upon them) do in the acts of mourning is expressing their grief and pain for what happened to Imam Hussein and his household on the day of Ashura. It is not something strange. It is, rather, an expression of the love and loyalty that Almighty Allah has ordered the Islamic nation to hold when He said: "Say (O , unto mankind): I ask of you no recompense for this, except love for the kinsfolk." (23:42). Sunnis and even non-Muslims participate in these commemorations since the loyalty to the Ahlulbait and loving them is not limited to the Shia.
These commemorations are not a tradition but it is an implementation of the orders of the Imams of the Ahlulbait to their followers, when they ordered them to hold such commemorations. It is narrated that Imam Ja'afar al-Sadiq (peace be upon him) said: "Enliven our commemoration. May Allah have mercy on those who enliven our commemorations". There are many other such narrations.
Yes, people and nations differ in their expressions of sadness, since the aim is to commemorate these tragic incidents, as means of sharing their grief and sorrow.

Shias mourn all the martyrs of Karbala. They remember their faithful dedication to sacrifice themselves with praise and respect, whether they were among the Ahlulbait or among the other companions. It is customary in some Shia communities to specify a day within the first ten days of Muharam to remember one of the martyrs of the companions of the Imam as well as a specific day for Abbas bin Ali (peace be upon him) and other members of the Ahlulbait.
The Ziyarats that were narrated from the Ahlulbait (peace be upon them), were specified for Imam Hussein (peace be upon him), while they also mention the companions with praise and Salam. It is natural that Shias will mainly mourn for Imam Hussein (peace be upon him) since he was the Imam and the rest were his followers and companions, in addition to his relation to the Messenger of Allah (peace be upon him and his holy progeny).

It is reported in religious texts not to wear black clothes, which suggests that it is disliked to do so. However, there is no doubt that it is preferable to wear black clothes to mourn what happened to Imam Hussain (peace be upon him) and his household and companions as this represents loyalty and love for the Ahlulbait (peace be upon them).

It is permissible to seek the blessings from whatever is related to the Prophet and the Imams (peace be upon them all). Seeking one's wishes from them is basically seeking intercession from them because of their high rank and closeness to Almighty Allah.

There is no doubt that we should commemorate the tragedy of Ashura and express our grief and sorrow over what happened to Imam Hussein (peace be upon him) and his family and companions; and the Imams (peace be upon them) emphasized this in a well-known manner that is recorded in the books of narrations.
However, differences have occurred in respect to some practices and rituals, and opinions have differed about them for some time.
Our advice to all the believers is that they should refer in this issue to those Maraji who they follow and fulfill the conditions of Taqleed, and they should distance themselves from conflicting and quarrelling over it, as a believer may commit a sin as a result of such arguing. Rather, it is best at times to leave such quarrelling when there is no benefit in it. This may even cause harm and lead to animosity and hostility between believers.

It is recommended to perform the acts of Arba'een on the 20th of Safar.

The greatest and the most tragic calamity to the friends of Allah and His righteous slaves happened on the day of Ashura. The son of the Messenger of Allah's daughter, his companions and family members including the infant child were killed and their bodies were scattered afterwards. The enemies attacked the tents in which the women and children gathered amongst which were the family members of the Messenger of Allah (peace be upon him).
The Imams of the Ahlulbait (peace be upon them) remembered this day by recounting the tragedy and expressing their sadness and grief. They also ordered their followers to do the same.
They also emphasized visiting the grave of the oppressed Imam Hussein (peace be upon him) on this day or recite the Ziyara if visiting his grave in Kerbala is not an option.

It is permissible to offer the believers such greetings on the tenth of Muharram. It is recommended for the believers to also offer their condolences to each other in the grief of the tragedy of Imam Hussein (peace be upon him) by saying: “Addhamallahu ujoorana bi-Musabil-Husain, wa Ja’alana wa Iyyakum minat-Talibeen bi Tharihim ma’a Waliyyihi al-Imam al-Mahdi min Aali Mohammad.”
The translation: “May Allah greaten your reward because the tragedy of Imam Hussein and make you and us amongst the seekers of his revenge with Imam Mahdi (may Allah hasten his reappearance)”.

Removing the turban is not obligatory but it could be – in some countries and based on certain conventional understandings – an expression of sadness and grief on what had happened to Imam Hussein and the Ahlulbait (peace be upon them) in Karbala when they faced numerous kinds of tragedies like their decapitation and placing their heads on spears.

All what believers do in order to express their grief and sadness and show their loyalty to the Ahlulbait (peace be upon them) are amongst the best of deeds. One should do so in such a way that the person is not harmed with a prohibited kind of harm.

It is not prohibited to study in the day or the night of Ashura.

Engaging in such activities on the night or day of Ashura is not in accordance to what has been narrated from the Imams (peace be upon them) in the exhortation to express grief and sadness over the tragedy suffered by Imam Hussain (peace be upon him), his household and companions on the day of Ashura.

These days are days of sadness and mourning for the Shia because of the tragedies that befell the Ahlulbait in this period of the year.

The abuse and slander against Shias will not stop even if they stopped performing Tatbeer, as it was continuing for centuries against them because of their commemoration of the tragedy of Ashura. The reason behind this is obvious: because it places the role of the Muslim caliphate into question since the authorities killed the grandson of the Prophet, Imam Hussien (peace be upon him), in such a terrible way only fifty years after the death of the founder of the religion, the Messenger of Allah (peace be upon him and his holy progeny).
The act of Tatbeer is not a general practice of the Shias on the day of Ashura since those who perform it is limited in number with respect to the population of Shias that commemorate the tragedy every year. From that, one can conclude that the slander against the Shia is not simply due to them performing Tatbeer.

As the act of Tatbeer and other kinds of symbolic actions are performed to propagate the right principal and express emotions towards it, then it is religiously recommended from this side. Yes, it can be prohibited because of secondary reasons, like if it causes prohibited harm, public disadvantage or other similar reasons. There is no set criterion for such prohibition as it differs from place to place and from time to time. It can also be different because of difference in personal perspectives.
We ask Allah Almighty to guide us and all devotees in all of that as we only depend on Him.

Religious rituals that Shias perform to mourn the tragedies of the Imams of the Ahlulbait (peace be upon them) and to present the oppressions that they suffered are recommended. However, it is obligatory to avoid what causes harm to the soul or the body. If it does not result in harm, it remains being recommended. One should avoid performing them in a place where the reputation of the Shias is damaged.

Mourning Imam Hussein (peace be upon him) as an expression of sadness and grief for the tragedy of Karbala is recommended. Some believers during such ceremonies bleed. The fact that our blood is Najis does not affect this expression of grief. Yes, if the body or clothes become Najis by blood, or any other Najasah, one should make it Tahir before offering the prayer.

It is prohibited to make the mosque Najis. It is not prohibited to make other places Najis, especially when they are going to be purified afterwards.